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

As pointed out at:
https://github.com/golang/go/issues/47916#issuecomment-908103163

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>
diff --git a/design/47916-parameterized-go-types.md b/design/47916-parameterized-go-types.md
index 2c195af..8c1cfc7 100644
--- a/design/47916-parameterized-go-types.md
+++ b/design/47916-parameterized-go-types.md
@@ -63,8 +63,8 @@
 ```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 { /* ... */ }