Fixes com_google_protobuf reference

[rodrigoaliste]

This fix enables direct copy-paste on the WORKSPACE file for bazel versions that don't handle strip_prefix automatically.

What type of PR is this?

Documentation

What does this PR do? Why is it needed?

Fixes the README reference of com_google_protobuf. Adds the strip_prefix so users don't spend minutes of their lives searching for the issue when getting the following error:

ERROR: error loading package '': Every .bzl file must have a corresponding package, but 
'@com_google_protobuf//:protobuf_deps.bzl' does not have one. Please create a BUILD file in the same or any parent directory. Note that this BUILD file does not need to do anything except exist.

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it! and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

[jayconrod]

Sorry, I can't review anything without a signed CLA.

Based on the description I'm guessing this is the same as #2442 and #2447 though.