fix a typo in calling the method noop

Author: nbayati

google/auth/credentials.py

@@ -310,7 +310,7 @@ def apply(self, headers, token=None):
         """Apply the token to the authentication header."""
         super().apply(headers, token)
         if self._trust_boundary is not None:
-            if self._has_no_op_trust_boundary:
+            if self._has_no_op_trust_boundary():
                 # STS expects an empty string if the trust boundary value is no-op.
                 headers["x-allowed-locations"] = ""
             else: