doc: use commit range as argument, not upstream/main

upstream/main depends on how you checked out the repo, it can be anything and user might have a different remote. Use <commit range> instead to make it more general. Fixes zephyrproject-rtos#96121 Signed-off-by: Anas Nashif <anas.nashif@intel.com>

Author: nashif

doc/contribute/guidelines.rst

@@ -607,7 +607,7 @@ before opening a new Pull Request:
 
 .. code-block:: bash
 
- ./scripts/ci/check_compliance.py -c upstream/main..
+ ./scripts/ci/check_compliance.py -c <commit range>
 
 .. note::
  On Windows if the .pl extension has not yet been associated with an