Update check_commit_msg_bug_field style

Other hooks have `field = '...'` in them.
For https://android-review.git.corp.google.com/c/platform/tools/repohooks/+/3295916/comment/e6779855_42bc8391/

Bug: 371923047
Test: rh/hooks_unittest.py
Change-Id: Ib8932f597df7f9988ac5c393c2845f5e5db3338d
Reviewed-on: https://android-review.googlesource.com/c/platform/tools/repohooks/+/3432026
Reviewed-by: Mike Frysinger <vapier@google.com>
Autosubmit: Li-Yu Yu <aaronyu@google.com>
Open-Source-Licensing: Lint 🤖 <ayeaye-gerrit@google.com>
Performance: CrystalBall Performance Presubmit <android-crystalball-presubmit-eng@google.com>
Presubmit-Verified: Treehugger Robot <android-test-infra-workplan-finisher@system.gserviceaccount.com>
Lint: Lint 🤖 <ayeaye-gerrit@google.com>

rh/hooks.py

diff --git a/rh/hooks.py b/rh/hooks.py
index ec8b69c..6bdf344 100644
--- a/rh/hooks.py
+++ b/rh/hooks.py
@@ -499,11 +499,12 @@
 
 def check_commit_msg_bug_field(project, commit, desc, _diff, options=None):
     """Check the commit message for a 'Bug:' or 'Fix:' line."""
+    field = 'Bug'
     regex = r'^(Bug|Fix): (None|[0-9]+(, [0-9]+)*)$'
     check_re = re.compile(regex)
 
     if options.args():
-        raise ValueError('commit msg Bug check takes no options')
+        raise ValueError(f'commit msg {field} check takes no options')
 
     found = []
     for line in desc.splitlines():
@@ -512,13 +513,13 @@
 
     if not found:
         error = (
-            'Commit message is missing a "Bug:" line.  It must match the\n'
+            f'Commit message is missing a "{field}:" line.  It must match the\n'
             f'following case-sensitive regex:\n\n    {regex}'
         )
     else:
         return None
 
-    return [rh.results.HookResult('commit msg: "Bug:" check',
+    return [rh.results.HookResult(f'commit msg: "{field}:" check',
                                   project, commit, error=error)]