[skip ci] Change to uppercase (#9467)

Signed-off-by: Nshiyu22 <shiyu.chen@zilliz.com>

CODE_REVIEW.md

@@ -76,4 +76,4 @@ Besides All the reviewer's responsibility listed above, Approvers should also ma
 
 * The variable names appearing in the source code need to be readable. Comments are necessary if it is a unusual abbreviations
 
-Thanks for [Code Review guide](https://github.com/pingcap/tidb/blob/master/code_review_guide.md) from Pingcap community.
+Thanks for [Code Review Guide](https://github.com/pingcap/tidb/blob/master/code_review_guide.md) from Pingcap community.