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

Changed files
+1 -1
nixos
modules
programs
+1 -1
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.
'';
};
};