go/types: fix lhs/rhs mixup in docs

Change-Id: Ifd51636c9254de51b8a21371d7507a9481bcca0a
Reviewed-on: https://go-review.googlesource.com/109142Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/api.go

@@ -309,7 +309,7 @@ func (tv TypeAndValue) Assignable() bool {
 }
 
 // HasOk reports whether the corresponding expression may be
-// used on the lhs of a comma-ok assignment.
+// used on the rhs of a comma-ok assignment.
 func (tv TypeAndValue) HasOk() bool {
 	return tv.mode == commaok || tv.mode == mapindex
 }