fix: [taxii21] Typo

Author: chrisr3d

misp_modules/modules/import_mod/taxii21.py

@@ -12,7 +12,7 @@
 import taxii2client
 import taxii2client.exceptions
 from misp_stix_converter import ExternalSTIX2toMISPParser, InternalSTIX2toMISPParser
-from misp_stix_converter.tools import _is_stix2_from_misp
+from misp_stix_converter.tools import is_stix2_from_misp
 from stix2.v20 import Bundle as Bundle_v20
 from stix2.v21 import Bundle as Bundle_v21