contributing.md: add version setting for docs

Author: jatcwang

CONTRIBUTING.md

@@ -53,6 +53,7 @@ After the version is set correctly:
 ```
 # Make the git worktree where gh-page branch will be checked out and docs copied over
 git fetch && git worktree add doc_worktree gh-pages
+sbtn 'set ThisBuild / version := "<latest released version>" '
 sbtn makeSite ghpagesSynchLocal 
 # Check doc changes are expected
 sbtn ghpagesPushSite