Fix docs build dataproc (#27563)

## Summary & Motivation

Fix docs build by adding missing dep

## Test plan

Ran `make sphinx-mdx` in docs and it worked

Author: smackesey

docs/tox.ini

@@ -75,7 +75,7 @@ deps =
   -e ../python_modules/libraries/dagstermill
   -e ../python_modules/libraries/dagster-aws
   -e ../python_modules/libraries/dagster-datahub
-  -e ../python_modules/libraries/dagster-gcp
+  -e ../python_modules/libraries/dagster-gcp[dataproc]
   -e ../python_modules/libraries/dagster-pyspark
   -e ../python_modules/libraries/dagster-ssh
   -e ../python_modules/libraries/dagster-duckdb