Bump protobuf from 3.17.3 to 3.18.3 in /docs

Bumps [protobuf](https://github.com/protocolbuffers/protobuf) from 3.17.3 to 3.18.3.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/main/generate_changelog.py)
- [Commits](protocolbuffers/protobuf@v3.17.3...v3.18.3)

---
updated-dependencies:
- dependency-name: protobuf
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

docs/requirements.txt

@@ -71,7 +71,7 @@ pluggy==0.13.1
 portalocker 
 poyo==0.5.0
 pre-commit==2.14.0
-protobuf==3.17.3
+protobuf==3.18.3
 py==1.10.0
 pyasn1==0.4.8
 pyasn1-modules==0.2.8