all: Correct spelling in comments

Skip-check: authors pr-build-mac

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3056

lib/osutil/atomic.go

@@ -20,7 +20,7 @@ var (
 )
 
 // An AtomicWriter is an *os.File that writes to a temporary file in the same
-// directory as the final path. On successfull Close the file is renamed to
+// directory as the final path. On successful Close the file is renamed to
 // it's final path. Any error on Write or during Close is accumulated and
 // returned on Close, so a lazy user can ignore errors until Close.
 type AtomicWriter struct {