docs: strip colons from help text

[robx]

This removes the (inconsistent) colons at the end of commands in the help text, to make the help text a bit more readable.

Before:

[...]
Commands:

init:                     Scaffold devenv.yaml, devenv.nix, and .envrc inside the current directory.
init TARGET:              Scaffold devenv.yaml, devenv.nix, and .envrc inside TARGET directory.
search NAME:              Search packages matching NAME in nixpkgs input.
shell:                    Activate the developer environment.
shell CMD [args]:         Run CMD with ARGS in the developer environment. Useful when scripting.
container [options] NAME  Generate a container for NAME. See devenv container --help and http://devenv.sh/containers
info:                     Print information about the current developer environment.
[...]

After:

[...]
Commands:

init                      Scaffold devenv.yaml, devenv.nix, and .envrc inside the current directory.
init TARGET               Scaffold devenv.yaml, devenv.nix, and .envrc inside TARGET directory.
search NAME               Search packages matching NAME in nixpkgs input.
shell                     Activate the developer environment.
shell CMD [args]          Run CMD with ARGS in the developer environment. Useful when scripting.
container [options] NAME  Generate a container for NAME. See devenv container --help and http://devenv.sh/containers
info                      Print information about the current developer environment.
[...]