link to chapter referred to

This made it look the the topic was covered in the chapter just before
the current one.

Author: tshepang

src/ty-fold.md

@@ -1,6 +1,6 @@
 # `TypeFoldable` and `TypeFolder`
 
-In the previous chapter we discussed instantiating binders. This must involves looking at everything inside of a `Early/Binder`
+In [a previous chapter], we discussed instantiating binders. This must involves looking at everything inside of a `Early/Binder`
 to find any usages of the bound vars in order to replace them. Binders can wrap an arbitrary rust type `T` not just a `Ty` so
 how do we implement the `instantiate` methods on the `Early/Binder` types.
 
@@ -102,3 +102,4 @@ calls
 [ty_for_param](https://github.com/rust-lang/rust/blob/75ff3110ac6d8a0259023b83fd20d7ab295f8dd6/src/librustc_middle/ty/subst.rs#L552-L587)
 and all that does is index into the list of substitutions with the index of the `Param`.
 
+[a previous chapter]: ty_module/instantiating_binders.md