Update HOWTO_RELEASE

HOWTO_RELEASE

@@ -1,7 +1,8 @@
 How to issue an OGGM release.
 
-1. Ensure your master branch is synced to upstream:
-       git pull upstream master
+ 1. Go to your local main repository (not the fork) and ensure your master branch is synced:
+       git checkout master
+       git pull
  2. Look over whats-new.rst and the docs. Make sure "What's New" is complete
     (check the date!) and add a brief summary note describing the release at the
     top.
@@ -20,20 +21,20 @@ How to issue an OGGM release.
     You will need to be listed as a package owner at
     https://pypi.python.org/pypi/oggm for this to work.
  8. Push your changes to master:
-      git push upstream master
-      git push upstream --tags
+      git push origin master
+      git push origin --tags
  9. Update the stable branch (used by ReadTheDocs) and switch back to master:
       git checkout stable
       git rebase master
-      git push upstream stable
+      git push origin stable
       git checkout master
     It's OK to force push to 'stable' if necessary.
     We also update the stable branch with `git cherrypick` for documentation
     only fixes that apply the current released version.
 10. Add a section for the next release (v.X.(Y+1)) to doc/whats-new.rst.
 11. Commit your changes and push to master again:
       git commit -a -m 'Revert to dev version'
-      git push upstream master
+      git push origin master
     You're done pushing to master!
 12. Issue the release on GitHub. Click on "Draft a new release" at
     https://github.com/OGGM/oggm/releases. Type in the version number, but