Button labels should use Title Case

It was erroneously set to "Enter passphrase" but never discovered
because Material uses ALL CAPS which hides the incorrect use of
lowercase on the second word.

Bug: 887932
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: Ia3ff5efefb992ca326ac3f94be3be13d78e1f831
Reviewed-on: https://chromium-review.googlesource.com/1252535
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595325}

ios/chrome/app/strings/ios_strings.grd

@@ -1539,7 +1539,7 @@ Handoff must also be enabled in the General section of Settings, and your device
         Encryption
       </message>
       <message name="IDS_IOS_SYNC_ENTER_PASSPHRASE" desc="Title displayed when the signed in user needs to enter its sync passphrase. [Length: 20em] [iOS only]">
-        Enter passphrase
+        Enter Passphrase
       </message>
       <message name="IDS_IOS_SYNC_ENTER_PASSPHRASE_BODY_WITH_EMAIL" desc="Instructions for the dialog where the user enters the passphrase.">
         Signed in as <ph name="EMAIL">$1<ex>johndoe@gmail.com</ex></ph>.