docs: update CONTRIBUTING.md to change PR base branch to main

Author: ppzqh

CONTRIBUTING.md

@@ -29,7 +29,7 @@ Before you submit your Pull Request (PR) consider the following guidelines:
 4. [Fork](https://docs.github.com/en/github/getting-started-with-github/fork-a-repo) the cloudwego/kitex repo.
 5. In your forked repository, make your changes in a new git branch:
     ```
-    git checkout -b my-fix-branch develop
+    git checkout -b my-fix-branch main
     ```
 6. Create your patch, including appropriate test cases. Please refer to [Go-UT](https://pkg.go.dev/testing#pkg-overview) for writing guides. [Go-Mock](https://github.com/golang/mock) is recommended to mock interface, please refer to internal/mocks/readme.md for more details, and [Mockey](https://github.com/bytedance/mockey) is recommended to mock functions, please refer to its readme doc for specific usage.
 7. Follow our [Style Guides](#code-style-guides).
@@ -39,7 +39,7 @@ Before you submit your Pull Request (PR) consider the following guidelines:
     ```
     git push origin my-fix-branch
     ```
-10. In GitHub, send a pull request to `kitex:develop` with a clear and unambiguous title.
+10. In GitHub, send a pull request to `kitex:main` with a clear and unambiguous title.
 
 ## Contribution Prerequisites
 - Our development environment keeps up with [Go Official](https://golang.org/project/).