doc/contributing: fix whitespace

doc/contributing/submitting-changes.chapter.md

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 ### Staging-next branch {#submitting-changes-staging-next-branch}
 
-The `staging-next` branch is for stabilizing mass-rebuilds submitted to the `staging` branch prior to merging them into `master`. Mass-rebuilds must go via the `staging` branch. It must only see non-breaking commits that are fixing issues blocking it from being merged into the `master ` branch.
+The `staging-next` branch is for stabilizing mass-rebuilds submitted to the `staging` branch prior to merging them into `master`. Mass-rebuilds must go via the `staging` branch. It must only see non-breaking commits that are fixing issues blocking it from being merged into the `master` branch.
 
 If the branch is already in a broken state, please refrain from adding extra new breakages. Stabilize it for a few days and then merge into master.