docs: Update .github/README.md

Co-authored-by: Oleksiy Kovyrin <oleksiy@kovyrin.net>

Author: joshka

.github/README.md

@@ -14,7 +14,7 @@ To perform this merge run:
 ```shell
 git remote add ci https://github.com/jonhoo/rust-ci-conf.git
 git fetch ci
-git merge --allow-unrelated-histories
+git merge --allow-unrelated-histories ci/main
 ```
 
 An overview of the files in this project is available at: