[iOS] Fix a typo in ios_strings_resources.grd

tools/grit/grit.py does not recognize "preprend" as a valid emit_type but
instead understand "prepend" which avoid the inclusion of atlres.h in the
generated file.

BUG=475514

Review URL: https://codereview.chromium.org/1113603003

Cr-Commit-Position: refs/heads/master@{#327323}

ios/chrome/app/strings/ios_strings_resources.grd

@@ -14,7 +14,7 @@ all other OS specific variables will be false.
       source_lang_id="en" enc_check="möl">
   <outputs>
     <output filename="grit/ios_strings_resources.h" type="rc_header">
-      <emit emit_type="preprend"></emit>
+      <emit emit_type="prepend"></emit>
     </output>
     <output filename="ios_strings_resources_am.pak" type="data_package" lang="am" />
     <output filename="ios_strings_resources_ar.pak" type="data_package" lang="ar" />