docs: add + fix eventarc ref docs

Author: Salakar

docs/generate.sh

@@ -85,6 +85,7 @@ TITLE="Firebase Python SDK for Cloud Functions"
 PY_MODULES='firebase_functions
             firebase_functions.core
             firebase_functions.db_fn
+            firebase_functions.eventarc_fn
             firebase_functions.https_fn
             firebase_functions.options
             firebase_functions.params

src/firebase_functions/eventarc_fn.py

@@ -35,6 +35,7 @@ def on_custom_event_published(
     Example:
 
     .. code-block:: python
+
       from firebase_functions import eventarc_fn
 
       @eventarc_fn.on_custom_event_published(