Minor edits of error message in fallback()

The error message was originally added in
https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4194897

Bug: 1340825
Change-Id: I6415fc81113f0d876067bd3e3e28f8c35cd164d9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4392152
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Wan-Teh Chang <wtc@google.com>
Reviewed-by: Junji Watanabe <jwata@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>

ninja.py

@@ -39,8 +39,8 @@ def fallback(ninja_args):
   print(
       'depot_tools/ninja.py: Could not find Ninja in the third_party of '
       'the current project, nor in your PATH.\n'
-      'Please take a following action to install Ninja.\n'
-      '- If your project has DEPS, Add a CIPD Ninja dependency to DEPS.\n'
+      'Please take one of the following actions to install Ninja:\n'
+      '- If your project has DEPS, add a CIPD Ninja dependency to DEPS.\n'
       '- Otherwise, add Ninja to your PATH *after* depot_tools.',
       file=sys.stderr)
   return 1