[skip ci]Fix CONTRIBUTING.md title format (#7918)

Signed-off-by: NWeida Zhu <weida.zhu@zilliz.com>

CONTRIBUTING.md

@@ -101,7 +101,7 @@ And we made the following changes based on the guide:
 - Commit message and PR description style: refer to [good commit messages](https://chris.beams.io/posts/git-commit)
 
 ### Format code
-####GO
+#### GO
 ```shell
 $ make fmt
 ```
@@ -125,12 +125,12 @@ $ make clang-format
 ## Run unit test with code coverage
 Before submitting your Pull Request, make sure you have run unit test, and your code coverage rate is >= 90%.
 
-###GO
+### GO
 ```shell
 $ go test -coverprofile fmtcoverage.html  ./internal/allocator
 ok  	github.com/milvus-io/milvus/internal/allocator 0.048s	coverage: 69.6% of statements
 ```
-###C++
+### C++
 Install lcov
 ```shell
 $ sudo apt-get install lcov