Specify the exact branch for git rebase

[bram2000]

PR Prelude

Thank you for working on SpaceVim! :)

Please complete these steps and check these boxes (by putting an x inside
the brackets) before filing your PR:

• I have read and understood SpaceVim's CONTRIBUTING document.
• I have read and understood SpaceVim's CODE_OF_CONDUCT document.
• I have included tests for the changes in my PR. If not, I have included a
  rationale for why I haven't.
• I understand my PR may be closed if it becomes obvious I didn't
  actually perform all of these steps.

Why this change is necessary and useful

The documentation contains a git command that fails with the error fatal: Needed a single revision invalid upstream 'upstream'. This is fixed by specifying the exact branch, rather than just the remote.

No test as this is purely a documentation change.

[codecov]

Codecov Report

Merging #1754 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1754   +/-   ##
=======================================
  Coverage   26.55%   26.55%           
=======================================
  Files         249      249           
  Lines       12405    12405           
=======================================
  Hits         3294     3294           
  Misses       9111     9111