Fix typos in the sheriffing document

BUG=skia:

NOTRY=true
DOCS_PREVIEW= https://skia.org/?cl=6470

Change-Id: Id80f5bb1a3f0279162b0397fd6822aec83eae322
Reviewed-on: https://skia-review.googlesource.com/6470
Reviewed-by: Stephan Altmueller <stephana@google.com>
Commit-Queue: Stephan Altmueller <stephana@google.com>
1 file changed