submitting-changes.chapter.md: avoid being specific

There is some doubt as to exactly how to enumerate all the merges from
one branch to another reliably.  In the meantime, let's be a little
more vague.

doc/contributing/submitting-changes.chapter.md

@@ -227,7 +227,7 @@ digraph {
 }
 ```
 
-[This GitHub Action](https://github.com/NixOS/nixpkgs/blob/master/.github/workflows/periodic-merge-6h.yml) brings changes from `master` to `staging-next` and from `staging-next` to `staging` every 6 hours; these are the blue arrows in the diagram above.  The purple arrows in the diagram above are done manually and much less frequently.  You can get an idea of how often these merges occur by looking at the `git-log` for `master` and searching for `from NixOS/staging-next` (for the upper purple arrow) or `staging-next into staging` (for the lower arrow).
+[This GitHub Action](https://github.com/NixOS/nixpkgs/blob/master/.github/workflows/periodic-merge-6h.yml) brings changes from `master` to `staging-next` and from `staging-next` to `staging` every 6 hours; these are the blue arrows in the diagram above.  The purple arrows in the diagram above are done manually and much less frequently.  You can get an idea of how often these merges occur by looking at the git history.
 
 
 ### Master branch {#submitting-changes-master-branch}