debugupgraderepo: minor documentation fix

When we specify `--no-changelog --no-manifest --no-filelog` we skip all revlog
optimization instead of all filelog optimization.

Also while I was here, for consistency, I did `optimisation` -> `optimization`
to make it consistent with rest of occurrences.
Note: I am not native speaker and I only changed it because of consistency. I
don't know which one is correct.

Differential Revision: https://phab.mercurial-scm.org/D9617

mercurial/debugcommands.py

@@ -3930,7 +3930,7 def debugupgraderepo(ui, repo, run=False
       * `--changelog`: optimize the changelog only
       * `--no-changelog --no-manifest`: optimize filelogs only
       * `--filelogs`: optimize the filelogs only
-      * `--no-changelog --no-manifest --no-filelogs`: skip all filelog optimisation
+      * `--no-changelog --no-manifest --no-filelogs`: skip all revlog optimizations
     """
     return upgrade.upgraderepo(
         ui, repo, run=run, optimize=set(optimize), backup=backup, **opts