buildbot: fix error message on swarming config check

Correct the error message for missing 'dimensions'.

Bug: None
Change-Id: Ib5db2b3e19c815a790cd9d37a40496d811747b7f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4994955
Reviewed-by: Ben Pastene <[email protected]>
Commit-Queue: Tatsuhisa Yamaguchi <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1220117}
diff --git a/testing/buildbot/generate_buildbot_json.py b/testing/buildbot/generate_buildbot_json.py
index 6b553334..509ad5b 100755
--- a/testing/buildbot/generate_buildbot_json.py
+++ b/testing/buildbot/generate_buildbot_json.py
@@ -2052,7 +2052,7 @@
     if 'swarming' in step_data:
       dimensions = step_data['swarming'].get('dimensions')
       if not dimensions:
-        raise BBGenErr('%s: %s / %s : os must be specified for all '
+        raise BBGenErr('%s: %s / %s : dimensions must be specified for all '
                        'swarmed tests' % (filename, builder, step_name))
       if not dimensions.get('os'):
         raise BBGenErr('%s: %s / %s : os must be specified for all '