fix: wrong extra name for xml validation

It would warn about missing xml validation and suggest installing
the json-validation extra instead of the xml one.

Fix by suggesting the correct extra.

Author: lazka

cyclonedx/validation/xml.py

@@ -37,7 +37,7 @@
 except ImportError as err:
     _missing_deps_error = MissingOptionalDependencyException(
         'This functionality requires optional dependencies.\n'
-        'Please install `cyclonedx-python-lib` with the extra "json-validation".\n'
+        'Please install `cyclonedx-python-lib` with the extra "xml-validation".\n'
     ), err