Merge #1603

1603: fixed workflow example in documentation r=mergify[bot] a=ghostwheel42

## What type of PR?

documentation

## What does this PR do?

fix error in workflow example. use "git remote add" to add upstream as remote.

### Related issue(s)

-/-

Co-authored-by: Alexander Graf <agraf@kevag-telekom.de>

docs/contributors/environment.rst

@@ -29,7 +29,7 @@ Short work flow example
 
   git clone https://github.com/<YOUR_USERNAME>/Mailu.git
   cd Mailu
-  git add remote upstream https://github.com/Mailu/Mailu.git
+  git remote add upstream https://github.com/Mailu/Mailu.git
   git checkout -b fix-something master
 
 Work on the code as desired. Before doing a commit, you should at least build