nix derivation add: Remove reference to "installable"

It doesn't operate on an installable, so don't mention it.

src/nix/derivation-add.md

@@ -3,7 +3,7 @@ R""(
 # Description
 
 This command reads from standard input a JSON representation of a
-[store derivation] to which an [*installable*](./nix.md#installables) evaluates.
+[store derivation].
 
 Store derivations are used internally by Nix. They are store paths with
 extension `.drv` that represent the build-time dependency graph to which