design/go2draft-type-parameters: fix typo

Change-Id: I720b9241aff65296222095d83ea967a176d07a42
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/239165
Reviewed-by: Robert Griesemer <gri@golang.org>
diff --git a/design/go2draft-type-parameters.md b/design/go2draft-type-parameters.md
index fe39f1c..65e42b8 100644
--- a/design/go2draft-type-parameters.md
+++ b/design/go2draft-type-parameters.md
@@ -2107,9 +2107,9 @@
 lists explicit types, and only types listed in the constraint would be
 permitted as cases.
 
-##### No way to express convertability
+##### No way to express convertibility
 
-The design has no way to express convertability between two different
+The design has no way to express convertibility between two different
 type parameters.
 For example, there is no way to write this function: