update UserListOptions docs

As of [fccbb9520b (diff-e754c06a3071f63f53cee61571452b4c),) `Users.List` method was updated to `Users.ListAll`. This commit updates the documentation for `UserListOptions` to reflect that change.

github/users.go

@@ -113,7 +113,7 @@ func (s *UsersService) Edit(user *User) (*User, *Response, error) {
 	return uResp, resp, err
 }
 
-// UserListOptions specifies optional parameters to the UsersService.List
+// UserListOptions specifies optional parameters to the UsersService.ListAll
 // method.
 type UserListOptions struct {
 	// ID of the last user seen