Fix ProxyPacUrl to have the correct deprecated key

Change-Id: Id55c73900c5b05153577f0190fe6158e3056627a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2457686Reviewed-by: Owen Min <zmin@chromium.org>
Commit-Queue: Chris Sharp <csharp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814837}

components/policy/resources/policy_templates.json

@@ -3998,7 +3998,7 @@
         'dynamic_refresh': True,
         'per_profile': True,
       },
-      'deprecate': True,
+      'deprecated': True,
       'example_value': 'https://internal.site/example.pac',
       'id': 24,
       'caption': '''URL to a proxy .pac file''',