patch 8.2.1751: using 2 where bool is expected may throw an error

Problem:    Using 2 where bool is expected may throw an error.
Solution:   Make this backwards compatible.
diff --git a/src/testdir/test_terminal2.vim b/src/testdir/test_terminal2.vim
index 87d1e35..64310f6 100644
--- a/src/testdir/test_terminal2.vim
+++ b/src/testdir/test_terminal2.vim
@@ -560,8 +560,8 @@
     endif
   endif
 
-  call assert_fails('call term_gettty(buf, 2)', 'E1023:')
-  call assert_fails('call term_gettty(buf, -1)', 'E1023:')
+  call assert_fails('call term_gettty(buf, 2)', 'E475:')
+  call assert_fails('call term_gettty(buf, -1)', 'E475:')
 
   call assert_equal('', term_gettty(buf + 1))