doc: remove reference to nix-shell

src/nix/shell.md

@@ -148,9 +148,8 @@ of Nixpkgs:
 
     #!nix shell --override-input nixpkgs github:NixOS/nixpkgs/eabc38219184cc3e04a974fe31857d8e0eac098d
 
-The examples above all used `-p` to get dependencies from Nixpkgs. You
-can also use a Nix expression to build your own dependencies. For
-example, the Python example could have been written as:
+You can also use a Nix expression to build your own dependencies. For example,
+the Python example could have been written as:
 
 ```python
 #! /usr/bin/env nix
@@ -166,6 +165,4 @@ python3.withPackages (ps: with ps; [ prettytable ])
 ```
 
 
-
-
 )""