docs: add nitpick ignore for internal multiprocessing.context.Process class

Signed-off-by: Todd Gamblin <tgamblin@llnl.gov>

lib/spack/docs/conf.py

@@ -206,6 +206,7 @@ def setup(sphinx):
     ("py:class", "TextIO"),
     ("py:class", "hashlib._Hash"),
     ("py:class", "concurrent.futures._base.Executor"),
+    ("py:class", "multiprocessing.context.Process"),
     # Spack classes that are private and we don't want to expose
     ("py:class", "spack.provider_index._IndexBase"),
     ("py:class", "spack.repo._PrependFileLoader"),