Update regex to match double backticks

Co-authored-by: Savannah Ostrowski <savannah@python.org>

Author: hugovk

Lib/argparse.py

@@ -538,7 +538,8 @@ def _apply_text_markup(self, text):
             return text
         text = _re.sub(
             r'`([^`]+)`',
-            rf'{t.prog_extra}\1{t.reset}',
+r'(`{1,2})([^`]+)\1',
+rf'{t.prog_extra}\2{t.reset}',
             text,
         )
         return text