[doc] Fetch upstream after adding it
We use the upstream remote later, e.g. in `git checkout -b new_branch
upstream/master`. For that to work, we need to fetch it first at least
once, otherwise the error message is rather confusing for newcomers to
git: "upstream/master is not a commit".
diff --git a/doc/ug/github_notes.md b/doc/ug/github_notes.md
index 3b7d904..c9815cb 100644
--- a/doc/ug/github_notes.md
+++ b/doc/ug/github_notes.md
@@ -54,6 +54,7 @@
$ git clone https://github.com/$GITHUB_USER/opentitan.git
$ cd opentitan
$ git remote add upstream https://github.com/lowRISC/opentitan.git
+$ git fetch upstream
$ git remote -v
```
