Follow-ups from docs#80: mixed canceled/cancelled spelling on the graceful-shutdown page

[FumingPower3925]

Minor points from the review of goceleris/docs#80 (companion of #803, fixes #753), left for a later docs pass.

• Mixed canceled / cancelled on src/content/docs/graceful-shutdown.md (CodeRabbit, docs#80 thread). At docs#80's head 283ed04 the page has 10 cancelled/Cancelled and 4 canceled, and none of the 4 is in docs#80's diff: line 23 (prose), line 57 (a comment inside a Go example), line 64 (prose), and line 83, which is a verbatim quote of celeris's StartWithContext godoc and should stay as quoted unless the godoc changes. The repo's path instructions ask for British spelling, so lines 23 and 64 (and the example comment at 57, if we want it) could become cancelled.