[MD settings] rename Search engine settings to Search engine

Alan felt that just the word 'Search' made the side nav
look like it was an option to search inside settings.  He
asked that the section name be changed to 'Search engine'.

BUG=589630

Review URL: https://codereview.chromium.org/1812523004

Cr-Commit-Position: refs/heads/master@{#381782}

chrome/app/settings_strings.grdp

@@ -562,7 +562,7 @@
 
   <!-- Search Page -->
   <message name="IDS_SETTINGS_SEARCH" desc="Name of the settings page which displays search engine preferences.">
-    Search
+    Search engine
   </message>
   <message name="IDS_SETTINGS_SEARCH_EXPLANATION" desc="Explanation for the search engine dropdown setting.">
     Search engine used when searching from the omnibox