docs: update samples dep to 'google-cloud-spanner==1.19.0' (#137)

Author: renovate-bot

samples/samples/requirements.txt

@@ -1,2 +1,2 @@
-google-cloud-spanner==1.17.1
+google-cloud-spanner==1.19.0
 futures==3.3.0; python_version < "3"