oh-my-zsh module: reword & fix manual build

docbook interpreted this as a tag and this sounded as if the option
defaulted to putting the cached directory into the nix store.

cc @Ma27 @fpletz

nixos/modules/programs/zsh/oh-my-zsh.nix

···
           type = types.str;
           description = ''
             Cache directory to be used by `oh-my-zsh`.
-            Default is /nix/store/<oh-my-zsh>/cache.
+            Without this option it would default to the read-only nix store.
           '';
         };
       };