patch 8.1.1955: tests contain typos

Problem:    Tests contain typos.
Solution:   Correct the typos. (Dominique Pelle)
diff --git a/src/testdir/test_tabpage.vim b/src/testdir/test_tabpage.vim
index e222067..c8ac114 100644
--- a/src/testdir/test_tabpage.vim
+++ b/src/testdir/test_tabpage.vim
@@ -58,7 +58,7 @@
   q
   "
   "
-  " Test for ":tab drop multi-opend-file" to keep current tabpage and window.
+  " Test for ":tab drop multi-opened-file" to keep current tabpage and window.
   new test1
   tabnew
   new test1