Fix sync docs incorrect merge

This copies the changes from an autogenerated section in the following commit:
https://github.com/rclone/rclone/commit/f2a15a174f8cdd21950b123f102ec9f9105098e1

cmd/sync/sync.go

@@ -40,7 +40,7 @@ errors at any point.  Duplicate objects (files with the same name, on
 those providers that support it) are also not yet handled.
 
 It is always the contents of the directory that is synced, not the
-directory so when source:path is a directory, it's the contents of
+directory itself. So when source:path is a directory, it's the contents of
 source:path that are copied, not the directory name and contents.  See
 extended explanation in the ` + "`" + `copy` + "`" + ` command above if unsure.