[CUPS] Update Printer Setup dialog headers.

chrome/app/settings_strings.grdp

 <?xml version="1.0" encoding="utf-8"?>
 <!-- Settings-specific strings (included from generated_resources.grd). -->
 <grit-part>
   <!-- About Page -->
   <message name="IDS_SETTINGS_ABOUT_PAGE_BROWSER_VERSION" desc="The text label describing the version of the browser, example: Version 57.0.2937.0 (Developer Build) unknown (64-bit)">
     Version <ph name="PRODUCT_VERSION">$1<ex>15.0.865.0</ex></ph> (<ph name="PRODUCT_CHANNEL">$2<ex>Developer Build</ex></ph>) <ph name="PRODUCT_MODIFIER">$3</ph> <ph name="PRODUCT_VERSION_BITS">$4</ph>
   </message>
   <if expr="chromeos">
     <message name="IDS_SETTINGS_ABOUT_PAGE_DETAILED_BUILD_INFO" desc="Label describing the section that shows detailed information about the current Chrome build.">
       Detailed build information
@@ skipping 807 matching lines @@
     <message name="IDS_SETTINGS_PRINTING_CUPS_PRINTERS_DETAILS" desc="Text for the drop down menu which allows the user to see the printer details.">
       Details
     </message>
     <message name="IDS_SETTINGS_PRINTING_CUPS_PRINTERS_REMOVE" desc="Text for the drop down menu which allows the user to remove the selected printer.">
       Remove
     </message>
     <message name="IDS_SETTINGS_PRINTING_CUPS_SEARCH_LABEL" desc="The placeholder text in the printer search field.">
       printer name
     </message>
     <message name="IDS_SETTINGS_PRINTING_CUPS_ADD_PRINTERS_NEARBY_TITLE" desc="Text for the title of the dialog that is used to add nearby printers.">
-      Add printers nearby
+      Add a nearby printer
     </message>
     <message name="IDS_SETTINGS_PRINTING_CUPS_ADD_PRINTERS_MANUALLY_TITLE" desc="Text for the title of the dialog that is used to manually add a printer instead of automatically finding ones nearby.">
-      Add printers manually
+      Add a printer manually
     </message>
     <message name="IDS_SETTINGS_PRINTING_CUPS_PRINTER_DETAILS_TITLE" desc="Text for the title of the Printer Details subpage.">
       Printer details
     </message>
     <message name="IDS_SETTINGS_PRINTING_CUPS_PRINTER_ADVANCED" desc="Label for the toggle control to show/hide the advanced details of the printer.">
       Advanced
     </message>
     <message name="IDS_SETTINGS_PRINTING_CUPS_PRINTER_ADVANCED_ACCESSIBILITY_LABEL" desc="Label for the button that toggles showing the advanced printer options for a particular printer. Only visible by screen reader software.">
       Show advanced printer options
     </message>
@@ skipping 1890 matching lines @@
       System
     </message>
     <message name="IDS_SETTINGS_SYSTEM_HARDWARE_ACCELERATION_LABEL" desc="Label for the checkbox that forces Chrome to render via hardware acceleration (GPU) when available.">
       Use hardware acceleration when available
     </message>
     <message name="IDS_SETTINGS_SYSTEM_PROXY_SETTINGS_BUTTON" desc="Text for the button that changes a user's network proxy settings.">
       Change proxy settings
     </message>
   </if>
 </grit-part>