C domain: Fix namespace-pop context (#11935)

Author: ferdnyc

sphinx/domains/c/__init__.py

@@ -374,7 +374,7 @@ def run(self) -> list[Node]:
             symbol = self.env.domaindata['c']['root_symbol']
         self.env.temp_data['c:parent_symbol'] = symbol
         self.env.temp_data['c:namespace_stack'] = stack
-        self.env.ref_context['cp:parent_key'] = symbol.get_lookup_key()
+        self.env.ref_context['c:parent_key'] = symbol.get_lookup_key()
         return []