protoreflect: fix typo in documentation

Update golang/protobuf#880

Change-Id: If896add4b98c071e7d45b2747c429bdc238e711b
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/184677
Reviewed-by: Herbie Ong <herbie@google.com>

reflect/protoreflect/type.go

@@ -48,7 +48,7 @@ type Descriptor interface {
 	// Support for this functionality is optional and may return nil.
 	Parent() Descriptor
 
-	// Index returns the the index of this descriptor within its parent.
+	// Index returns the index of this descriptor within its parent.
 	// It returns 0 if the descriptor does not have a parent or if the parent
 	// is unknown.
 	Index() int