Update "Learn more" link in accessibility detailed view

"Learn more" link in accessibility detailed view is used to point to the
how to turn on accessibility features page, it is now updated to point to
the general help page of accessibility.

BUG=638439

Review-Url: https://codereview.chromium.org/2366713003
Cr-Commit-Position: refs/heads/master@{#420733}

chrome/common/url_constants.cc

@@ -432,7 +432,7 @@ const char kChromeHelpViaWebUIURL[] =
 
 #if defined(OS_CHROMEOS)
 const char kChromeAccessibilityHelpURL[] =
-    "https://support.google.com/chromebook/?p=accessibility_menu";
+    "https://support.google.com/chromebook/topic/6323347";
 const char kChromeAccessibilitySettingsURL[] =
     "/chromevox/background/options.html";
 const char kChromePaletteHelpURL[] =