runtime(doc): update vim90 to vim91 in docs

closes: #15747

Signed-off-by: mikoto2000 <mikoto2000@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/todo.txt b/runtime/doc/todo.txt
index e0ebe7e..3cfe59a 100644
--- a/runtime/doc/todo.txt
+++ b/runtime/doc/todo.txt
@@ -86,8 +86,8 @@
 
 Errors when running tests with valgrind:
 - test_codestyle.vim:  e.g.:
-    command line..script /home/mool/vim/vim90/src/testdir/runtest.vim[569]..function RunTheTest[52]..Test_test_files line 6: keycode_check.vim: space before tab: Expected 0 but got 7
-    command line..script /home/mool/vim/vim90/src/testdir/runtest.vim[569]..function RunTheTest[52]..Test_test_files line 10: setup.vim: trailing white space: Expected 0 but got 23
+    command line..script /home/mool/vim/vim91/src/testdir/runtest.vim[569]..function RunTheTest[52]..Test_test_files line 6: keycode_check.vim: space before tab: Expected 0 but got 7
+    command line..script /home/mool/vim/vim91/src/testdir/runtest.vim[569]..function RunTheTest[52]..Test_test_files line 10: setup.vim: trailing white space: Expected 0 but got 23
 - test_gui.vim:
     Found errors in Test_gui_mouse_event():