patch 8.1.1955: tests contain typos

Problem:    Tests contain typos.
Solution:   Correct the typos. (Dominique Pelle)
diff --git a/src/testdir/test_quickfix.vim b/src/testdir/test_quickfix.vim
index 5aad587..7a1f96d 100644
--- a/src/testdir/test_quickfix.vim
+++ b/src/testdir/test_quickfix.vim
@@ -1667,7 +1667,7 @@
   call assert_equal(1, bufwinnr('Xqftestfile3'))
 
   " If only quickfix window is open in the current tabpage, jumping to an
-  " entry with 'switchubf' set to 'usetab' should search in other tabpages.
+  " entry with 'switchbuf' set to 'usetab' should search in other tabpages.
   enew | only
   set switchbuf=usetab
   tabedit Xqftestfile1