docs: fix a typo in UPGRADING.md related to static_discovery

[parthea]

See the code below

google-api-python-client/googleapiclient/discovery.py

Lines 270 to 274 in cb89554

	if static_discovery is None:
	if discoveryServiceUrl is None:
	static_discovery = True
	else:
	static_discovery = False

This PR proposes that we change the wording in UPGRADING.md from

if `static_discovery` is not
specified, the default value for `static_discovery` will be `True` when
the `discoveryServiceUrl` argument of `discovery.build()` is provided.

to

if `static_discovery` is not
specified, the default value for `static_discovery` will be `False` when
the `discoveryServiceUrl` argument of `discovery.build()` is provided.