Tweak wording to be consistent with nearby strings.

Bug: 900293
Change-Id: Ibdb90ce9d2777311a1f67cd992f3ae326a9e1f01
Reviewed-on: https://chromium-review.googlesource.com/c/1338463Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Sidney San Martín <sdy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608807}

chrome/app/settings_strings.grdp

@@ -595,7 +595,7 @@
       Pressing Tab on a webpage highlights links, as well as form fields
     </message>
     <message name="IDS_SETTINGS_WARN_BEFORE_QUITTING_PREF" desc="The documentation string of the 'Warn Before Quitting' preference which is also available in the Chrome app menu.">
-      Show a warning before quitting with ⌘Q
+      Show warning before quitting with ⌘Q
     </message>
   </if>