docs: make name 'Billing Budget'

Author: busunkim96

README.rst

@@ -168,9 +168,9 @@ The following client libraries have **alpha** support:
 .. _HappyBase README: https://github.com/googleapis/google-cloud-python-happybase
 .. _HappyBase Documentation: https://google-cloud-python-happybase.readthedocs.io/en/latest/
 
-.. _Google Cloud Billing Budgets: https://pypi.org/project/google-cloud-billing-budget
-.. _Billing Budgets README: https://github.com/googleapis/google-cloud-python/blob/master/billingbudgets
-.. _Billing Budgets Documentation: https://googleapis.dev/python/billingbudgets/latest
+.. _Google Cloud Billing Budget: https://pypi.org/project/google-cloud-billing-budget
+.. _Billing Budget README: https://github.com/googleapis/google-cloud-python/blob/master/billingbudgets
+.. _Billing Budget Documentation: https://googleapis.dev/python/billingbudgets/latest
 
 .. _Google Cloud Build: https://pypi.org/project/google-cloud-build/
 .. _Cloud Build README: https://github.com/googleapis/google-cloud-python/cloudbuild