Fix wrong variable in exclude filter error message

[RajanChavada]

Link to issue or discussion

Found during code review — copy-paste bug in css/query.py. No existing issue; the fix is a single character change with no ambiguity.

What this PR does

When exclude= is passed to a query and the selector fails to parse, the raised InvalidQueryFormat message incorrectly references filter (and its value) instead of exclude:

# before (L108)
raise InvalidQueryFormat(f"Unable to parse filter {filter!r} as query")

# after
raise InvalidQueryFormat(f"Unable to parse exclude {exclude!r} as query")

The filter branch directly above (L102) is correct — this was a copy-paste of that line without updating the variable name.

Testing

All 16 existing tests/test_query.py tests pass. No logic changed, error message string only.