doc/submitting-changes: should -> must

To me, as a native English speaker, this doesn't change the meaning of
the sentence at all.  But to a non-native speaker, this can read like
the staging-next rules are only recommendations.  Let's make this
clearer.

1 files changed, 1 insertions, 1 deletions

diff --git a/doc/contributing/submitting-changes.chapter.md b/doc/contributing/submitting-changes.chapter.md
index e360dba5abd..b462caf8670 100644
--- a/doc/contributing/submitting-changes.chapter.md
+++ b/doc/contributing/submitting-changes.chapter.md
@@ -238,7 +238,7 @@ The `staging` branch is a development branch where mass-rebuilds go. It should o
 
 ### 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 should go via the `staging` branch. It should 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.