Printer settings: Fix casing on add printer button

Screenshots:

Before:
• http://shortn/_z0AhPafkbB

After:
• http://shortn/_GmWk2Ou5Gb

BUG=1066273
TEST=manual inspection

Change-Id: Ic1a47ff8f02b3c30f4800040c6f176ae41d79915
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2356670Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Reviewed-by: Jimmy Gong <jimmyxgong@chromium.org>
Commit-Queue: Michael Checo <michaelcheco@google.com>
Cr-Commit-Position: refs/heads/master@{#798293}

chrome/app/os_settings_strings.grdp

@@ -1577,7 +1577,7 @@
     Set up or manage CUPS printers. <ph name="LINK_BEGIN">&lt;a&gt;</ph>Learn more<ph name="LINK_END">&lt;/a&gt;</ph>
   </message>
   <message name="IDS_SETTINGS_PRINTING_CUPS_PRINTERS_ADD_PRINTER" desc="In CUPS printing settings subpage, text for the link adding a new CUPS printer.">
-    Add Printer
+    Add printer
   </message>
     <message name="IDS_SETTINGS_PRINTING_CUPS_PRINTERS_ADD_DETECTED_OR_NEW_PRINTER" desc="In CUPS printing settings subpage, explanatory text for the nearby printers list.">
     Save detected printers to your profile, or add a new printer. <ph name="LINK_BEGIN">&lt;a&gt;</ph>Learn more<ph name="LINK_END">&lt;/a&gt;</ph>

chrome/app/os_settings_strings_grdp/IDS_SETTINGS_PRINTING_CUPS_PRINTERS_ADD_PRINTER.png.sha1

+2d222a5fe231ab9ec48b41e637af5a683311f8cb
\ No newline at end of file