patch 8.2.2723: assignment test fails

Problem:    Assignment test fails.
Solution:   Adjust error number.
diff --git a/src/testdir/test_let.vim b/src/testdir/test_let.vim
index 2a37c19..1b8f74b 100644
--- a/src/testdir/test_let.vim
+++ b/src/testdir/test_let.vim
@@ -299,7 +299,7 @@
   call assert_fails('let l[:][0] = [5]', 'E708:')
   let d = {'k' : 4}
   call assert_fails('let d.# = 5', 'E488:')
-  call assert_fails('let d.m += 5', 'E734:')
+  call assert_fails('let d.m += 5', 'E716:')
   call assert_fails('let m = d[{]', 'E15:')
   let l = [1, 2]
   call assert_fails('let l[2] = 0', 'E684:')