Fix --editable (mode) option given --user as arg (#648).

Reviewed-on: https://projects.torsion.org/borgmatic-collective/borgmatic/pulls/650

docs/how-to/develop-on-borgmatic.md

@@ -26,7 +26,7 @@ make sure your changes work.
 
 ```bash
 cd borgmatic/
-pip3 install --editable --user .
+pip3 install --user --editable .
 ```
 
 Note that this will typically install the borgmatic commands into