docs: Fix more capitalization issues.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

Author: andersk

.github/pull_request_template.md

@@ -1,10 +1,10 @@
 <!-- What's this PR for?  (Just a link to an issue is fine.) -->
 
 
-**Testing Plan:** <!-- How have you tested? -->
+**Testing plan:** <!-- How have you tested? -->
 
 
-**GIFs or Screenshots:** <!-- If a UI change.  See:
+**GIFs or screenshots:** <!-- If a UI change.  See:
   https://zulip.readthedocs.io/en/latest/tutorials/screenshot-and-gif-software.html
   -->
 

.github/workflows/cancel-previous-runs.yml

@@ -1,4 +1,4 @@
-name: Cancel Previous Runs
+name: Cancel previous runs
 on: [push, pull_request]
 
 defaults:
@@ -7,7 +7,7 @@ defaults:
 
 jobs:
   cancel:
-    name: Cancel Previous Runs
+    name: Cancel previous runs
     runs-on: ubuntu-latest
     timeout-minutes: 3