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

angular/protractor@fcd973b#diff-f3b56000093113bd3bfb6c9c05e7e945 splits the overview doc into
multiple files, and removes overview.md from the repository entirely, making the current link a 404.

This CL points the link to the new getting-started document rather than the overview, and avoids
linking to a missing document.

Closes #8595