fix broken reference

Reference to a mathlib file which no longer exists has been removed, and replaced by a more user-friendly example of an equivalence relation.

docs/theories/relations.md

@@ -30,4 +30,7 @@ defined in `init/data/quot.lean` . The online document
 *Theorem Proving In Lean* has 
 [quite a good section about setoids and quotients](https://leanprover.github.io/theorem_proving_in_lean/axioms_and_computation.html#quotients)
 
-A nice mathematical example can be found in `linear_algebra/quotient_module`.
+A nice mathematical example can be found in `linear_algebra/basic.lean`. The definition of `quotient_rel`
+says that `x` and `y` are related iff `x - y` is in a fixed submodule `p`, and then it is proved
+(rather succinctly) that this is an equivalence relation, and the structure of a module is put
+on the equivalence classes. An example more readable for beginners is [here](https://github.com/kbuzzard/xena/blob/master/Examples/zmod37.lean), where the structure of a ring is put on the quotient space Z/37Z.