Copy edit SDK section.

chrome/common/extensions/docs/templates/json/chrome_sidenav.json

 [
   {
     "title": "Devtools",
     "items": [
       {
         "title": "Learn Basics",
         "items": [
           {
             "title": "Overview",
             "href": "/devtools/index"
@@ skipping 685 matching lines @@
                 "title": "What Is Native Client?",
                 "href": "/native-client/overview"
               },
               {
                 "title": "NaCl and PNaCl",
                 "href": "/native-client/nacl-and-pnacl"
               }
             ]
           },
           {
-            "title": "SDK",
+            "title": "Download the SDK",
             "href": "/native-client/sdk/download",
             "items": [
               {
-                "title": "Download SDK",
+                "title": "Download the Native Client SDK",
                 "href": "/native-client/sdk/download"
               },
               {
                 "title": "Examples",
                 "href": "/native-client/sdk/examples"
               },
               {
-                "title": "SDK Release Notes",
+                "title": "Release Notes",
                 "href": "/native-client/sdk/release-notes"
               }
             ]
           },
           {
             "title": "Tutorial",
             "href": "/native-client/devguide/tutorial/tutorial-part1",
             "items": [
               {
                 "title": "Part 1: Simple PNaCl Web App",
@@ skipping 272 matching lines @@
                 "title": "Articles",
                 "href": "/webstore/articles"
               }
             ]
           }
         ]
       }
     ]
   }
 ]