design/47916-parameterized-go-types.md: fix typo in TParamList methods

As pointed out at:
golang/go#47916 (comment)

Fix the receiver for TParamList methods.

Change-Id: Ic26e859a0830447c2926828e29d5f27bf2c5637f
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/346089
Reviewed-by: Robert Findley <rfindley@google.com>

design/47916-parameterized-go-types.md

@@ -63,8 +63,8 @@ Type parameters are considered identical (as reported by the `Identical` functio
 ```go
 type TParamList struct { /* ... */ }
 
-func (*TypeList) Len() int
-func (*TypeList) At(i int) *TypeParam
+func (*TParamList) Len() int
+func (*TParamList) At(i int) *TypeParam
 
 type TypeList struct { /* ... */ }