Fix protobuf repository's owner name on README.md

[micnncim]

On README.md, it seems have a wrong owner name for protobuf repository as below.
google/protobuf -> protocolbuffers/protobuf
Currently, when we click google/protobuf link, redirected to protocolbuffers/protobuf. Is this just as one intended?

[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 (e.g. 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.

[codecov-io]

Codecov Report

Merging #814 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #814   +/-   ##
======================================
  Coverage    53.3%   53.3%           
======================================
  Files          30      30           
  Lines        3371    3371           
======================================
  Hits         1797    1797           
  Misses       1399    1399           
  Partials      175     175

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 09679ff...7bb067d. Read the comment docs.

[micnncim]

I signed it!

[googlebot]

CLAs look good, thanks!

[johanbrandhorst]

Thanks for your contribution!