fix: update outdated command instructions

`gt rig add` no longer takes in a flag for --remote its just the
parameter of the github url.

`gt crew add` requires a --rig but the instructions don't specify the
flag

docs/INSTALLING.md

@@ -111,7 +111,7 @@ gt install ~/gt
 
 ```bash
 # Add your first project
-gt rig add myproject --remote=https://github.com/you/repo.git
+gt rig add myproject https://github.com/you/repo.git
 
 # This clones the repo and sets up:
 #   ~/gt/myproject/