MD Settings: Tweak "no certificates" string based on feedback.

chrome/app/settings_strings.grdp

 <?xml version="1.0" encoding="utf-8"?>
 <!-- Settings-specific strings (included from generated_resources.grd). -->
 <grit-part>
   <!-- Main Page -->
   <if expr="chromeos">
     <message name="IDS_SETTINGS_SECONDARY_USER_BANNER" desc="Banner displayed in settings page when the user is secondary in a multi-profile session.">
       Some settings belonging to <ph name="PRIMARY_EMAIL">$1<ex>john@google.com</ex></ph> are being shared with you. These settings only affect your account when using multiple sign-in.
     </message>
   </if>
 
@@ skipping 618 matching lines @@
     <message name="IDS_SETTINGS_CERTIFICATE_MANAGER_YOUR_CERTIFICATES_DESCRIPTION" desc="Label for your certificates subtitle tab in certificate manager.">
       You have certificates from these organizations that identify you
     </message>
     <message name="IDS_SETTINGS_CERTIFICATE_MANAGER_SERVERS" desc="Label for the servers tab in certificate manager.">
       Servers
     </message>
     <message name="IDS_SETTINGS_CERTIFICATE_MANAGER_SERVERS_DESCRIPTION" desc="Label for servers subtitle tab in certificate manager.">
       You have certificates on file that identify these servers
     </message>
     <message name="IDS_SETTINGS_CERTIFICATE_MANAGER_NO_CERTIFICATES" desc="Message displayed by the certificate manager, indicating that there are no certificates for a given certificate category.">
-      You have no certificates
+      You have no certificates in this category
     </message>
     <message name="IDS_SETTINGS_CERTIFICATE_MANAGER_AUTHORITIES" desc="Label for the authorities tab in certificate manager.">
       Authorities
     </message>
     <message name="IDS_SETTINGS_CERTIFICATE_MANAGER_AUTHORITIES_DESCRIPTION" desc="Label for authorities subtitle tab in certificate manager.">
       You have certificates on file that identify these certificate authorities
     </message>
     <message name="IDS_SETTINGS_CERTIFICATE_MANAGER_VIEW" desc="Label for view button in certificate manager.">
       View
     </message>
@@ skipping 2313 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>