README.md: fix typo, should be: spack repo add

* Thanks to Natalia Tilinina for reporting the error.

Author: harshula

README.md

@@ -26,7 +26,7 @@ git clone [email protected]:ACCESS-NRI/spack-packages.git $PACKAGE_PATH/spack-packa
 ```
 and then add the location of the repository to your spack instance
 ```
-git repo add $PACKAGE_PATH/spack-packages
+spack repo add $PACKAGE_PATH/spack-packages
 ```
 and then confirm it is has been added correctly:
 ```