Amend phrasing

This may seem pedantic, but I found the part saying that `--run` "_executes the given Bash command_" command misleading, especially if someone new to Linux shells would read it.

This suggestion is far from being perfect, but anything else I came up with seemed either too long-winded and/or out of place.

Thank you for these great write-ups!

source/tutorials/first-steps/ad-hoc-shell-environments.md

@@ -179,7 +179,7 @@ git version 2.33.1
 
 There are three things going on here:
 
-1. `--run` executes the given [Bash](https://www.gnu.org/software/bash/) command within the Nix shell and exits when it's done.
+1. `--run` executes the given command within the resulting Nix shell (which is a [Bash](https://www.gnu.org/software/bash/) sub-shell) and exits when it's done.
 
    You can use this with `nix-shell` whenever you want to quickly run a program you don't have installed on your machine.