aosp: Use main branch of git-repo

Author: taoky

docs/aosp.md

@@ -43,7 +43,7 @@ mkdir ~/bin
 PATH=~/bin:$PATH
 curl https://storage.googleapis.com/git-repo-downloads/repo > ~/bin/repo
 ## 如果上述 URL 不可访问，可以用下面的：
-## curl -sSL  'https://gerrit-googlesource.proxy.ustclug.org/git-repo/+/master/repo?format=TEXT' |base64 -d > ~/bin/repo
+## curl -sSL 'https://gerrit-googlesource.proxy.ustclug.org/git-repo/+/main/repo?format=TEXT' | base64 -d > ~/bin/repo
 chmod a+x ~/bin/repo
 ```