docs(guide/e2e-testing): fix the link to protractor's getting started

[NicolaeNMV]

The current link is broken as the protractor commit angular/protractor@fcd973b#diff-f3b56000093113bd3bfb6c9c05e7e945 that splits the overview doc into multiple files.

[mary-poppins]

I'm sorry, but I wasn't able to verify your Contributor License Agreement (CLA) signature. CLA signature is required for any code contributions to AngularJS.

Please sign our CLA and ensure that the CLA signature email address and the email address in this PR's commits match.

If you signed the CLA as a corporation, please let us know the company's name.

Thanks a bunch!

PS: If you signed the CLA in the past then most likely the email addresses don't match. Please sign the CLA again or update the email address in the commit of this PR.
PS2: If you are a Googler, please sign the CLA as well to simplify the CLA verification process.

[mary-poppins]

CLA signature verified! Thank you!

Someone from the team will now triage your PR and it will be processed based on the determined priority (doc updates and fixes with tests are prioritized over other changes).

[caitp]

LGTM, @juliemr it would be really cool if we preprocessed the markdown files and served them on a non-github website (or maybe even a gh-pages branch or something), so that there's something nicer to look at.

But I think this is good for now :>