github: fix some grammar issues in the pull request template (pingcap…

…#48798)

.github/pull_request_template.md

@@ -13,7 +13,7 @@ PR Title Format:
 
 Please create an issue first to describe the problem.
 
-There MUST be one line starting with "Issue Number:  " and 
+There MUST be one line starting with "Issue Number:  " and
 linking the relevant issues via the "close" or "ref".
 
 For more info, check https://pingcap.github.io/tidb-dev-guide/contribute-to-tidb/contribute-code.html#referring-to-an-issue.
@@ -24,7 +24,7 @@ Issue Number: close #xxx
 
 Problem Summary:
 
-### What is changed and how it works?
+### What changed and how does it work?
 
 ### Check List
 
@@ -35,7 +35,7 @@ Tests <!-- At least one of them must be included. -->
 - [ ] Manual test (add detailed scripts or steps below)
 - [ ] No need to test
   > - [ ] I checked and no code files have been changed.
-  > <!-- Or your custom  "No need to test" reasons -->  
+  > <!-- Or your custom  "No need to test" reasons -->
 
 Side effects