fix(jenkins): reset baseUrl in protractor conf

Commit 22b817ec11f7ab1a81342a4b60acd644a3f2a8c3 changed the url
used by protractor in all docs tests to prepend "build/docs", which
was already set to the `baseUrl` in `protractor-jenkins.conf`. This
commit just changes the protractor config's `baseUrl` to adapt
to the changes in the spec files.

Closes #9783