[docs] Tweak update to Phabricator docs about setting repository for diffs uploaded via web

Summary:
In D41919, I missed that there was a *second* step when uploading
diffs via web where the repository should be specified.

Reviewers: asb, probinson

Reviewed By: asb

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D41956

llvm-svn: 322375