Update colcon mixin add command URL (#6877) (#6884)

(cherry picked from commit 65edf65)

Signed-off-by: Janosch Machowinski <jmachowinski@users.noreply.github.com>
Co-authored-by: Janosch Machowinski <jmachowinski@users.noreply.github.com>

Author: mergify[bot]

source/Installation/Alternatives/Ubuntu-Development-Setup.rst

@@ -94,7 +94,7 @@ Install colcon mixins
 
 .. code-block:: console
 
-   $ colcon mixin add default https://github.com/colcon/colcon-defaults/raw/master/index.yaml
+   $ colcon mixin add default https://github.com/colcon/colcon-mixin-repository/raw/master/index.yaml
    $ colcon mixin update default
 
 Build the code in the workspace