Opt-out from regendoc for problematic example (#8810)

* Opt-out from regendoc for problematic example

See #8807

* Add comment

doc/en/how-to/skipping.rst

@@ -369,9 +369,12 @@ Here is a simple test file with the several usages:
 
 Running it with the report-on-xfail option gives this output:
 
+.. FIXME: Use $ instead of ! again to reenable regendoc once it's fixed:
+   https://github.com/pytest-dev/pytest/issues/8807
+
 .. code-block:: pytest
 
-    example $ pytest -rx xfail_demo.py
+    ! pytest -rx xfail_demo.py
     =========================== test session starts ============================
     platform linux -- Python 3.x.y, pytest-6.x.y, py-1.x.y, pluggy-0.x.y
     cachedir: $PYTHON_PREFIX/.pytest_cache