fix: add multiprocessing.rst to synthool excludes (googleapis#186)

Author: andrewsg

synth.py

@@ -28,6 +28,6 @@
 # Add templated files
 # ----------------------------------------------------------------------------
 templated_files = common.py_library(cov_level=99)
-s.move(templated_files, excludes=["noxfile.py"])
+s.move(templated_files, excludes=["noxfile.py, docs/multiprocessing.rst"])
 
 s.shell.run(["nox", "-s", "blacken"], hide_output=False)