Change "Always Show Shelf" string to sentence case.

Bug: 881157
Test: manual
Change-Id: I809b94d822a40432e108519d07c27787a71f86be
Reviewed-on: https://chromium-review.googlesource.com/1211111Reviewed-by: Vladislav Kaznacheev <kaznacheev@chromium.org>
Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Commit-Queue: Vladislav Kaznacheev <kaznacheev@chromium.org>
Commit-Queue: Alex Newcomer <newcomer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589253}

ash/ash_strings.grd

@@ -146,7 +146,7 @@ This file contains the strings for ash.
         Autohide shelf
       </message>
       <message name="IDS_ASH_SHELF_CONTEXT_MENU_ALWAYS_SHOW_SHELF" desc="Title of the menu item in the context menu for always showing the shelf">
-        Always Show Shelf
+        Always show shelf
       </message>
       <message name="IDS_ASH_SHELF_CONTEXT_MENU_POSITION" desc="Title of the menu item in the context menu for aligning the shelf">
         Shelf position