Update the protobuf version in the doc (#387)

Signed-off-by: SimFG <bang.fu@zilliz.com>

Author: SimFG

docs/requirements.txt

@@ -14,4 +14,4 @@ sphinx_copybutton
 pydata-sphinx-theme
 m2r2
 sphinx_toolbox
-protobuf==3.20.2
+protobuf==3.20.0