make more readable

Author: tshepang

src/doc/rustc-dev-guide/src/ty-fold.md

@@ -1,7 +1,7 @@
 # `TypeFoldable` and `TypeFolder`
 
 In [a previous chapter], we discussed instantiating binders. This 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
+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.
 
 The answer is a couple of traits: