Change the default branch in the agit docs (#27621) (#27623)

Backport #27621 by @lng2020

It's main now.
relevant #27579

Co-authored-by: Nanguan Lin <70063547+lng2020@users.noreply.github.com>

docs/content/usage/agit-support.en-us.md

@@ -26,7 +26,7 @@ This can be done by pushing to the branch followed by a specific refspec (a loca
 The following example illustrates this:
 
 ```shell
-git push origin HEAD:refs/for/master
+git push origin HEAD:refs/for/main
 ```
 
 The command has the following structure:
@@ -42,8 +42,8 @@ The command has the following structure:
   - `description`: The PR description
   - `force-push`: confirm force update the target branch
 
-Here's another advanced example for creating a new PR targeting `master` with `topic`, `title`, and `description`:
+Here's another advanced example for creating a new PR targeting `main` with `topic`, `title`, and `description`:
 
 ```shell
-git push origin HEAD:refs/for/master -o topic="Topic of my PR" -o title="Title of the PR" -o description="# The PR Description\nThis can be **any** markdown content.\n- [x] Ok"
+git push origin HEAD:refs/for/main -o topic="Topic of my PR" -o title="Title of the PR" -o description="# The PR Description\nThis can be **any** markdown content.\n- [x] Ok"
 ```

docs/content/usage/agit-support.zh-cn.md

@@ -26,7 +26,7 @@ Agit 允许在推送代码到远程仓库时创建 PR（合并请求）。
 下面的示例说明了这一点：
 
 ```shell
-git push origin HEAD:refs/for/master
+git push origin HEAD:refs/for/main
 ```
 
 该命令的结构如下：
@@ -42,8 +42,8 @@ git push origin HEAD:refs/for/master
   - `description`：PR 的描述
   - `force-push`：确认强制更新目标分支
 
-下面是另一个高级示例，用于创建一个以 `topic`、`title` 和 `description` 为参数的新 PR，目标分支是 `master`：
+下面是另一个高级示例，用于创建一个以 `topic`、`title` 和 `description` 为参数的新 PR，目标分支是 `main`：
 
 ```shell
-git push origin HEAD:refs/for/master -o topic="Topic of my PR" -o title="Title of the PR" -o description="# The PR Description\nThis can be **any** markdown content.\n- [x] Ok"
+git push origin HEAD:refs/for/main -o topic="Topic of my PR" -o title="Title of the PR" -o description="# The PR Description\nThis can be **any** markdown content.\n- [x] Ok"
 ```