patch 8.1.1955: tests contain typos

Problem:    Tests contain typos.
Solution:   Correct the typos. (Dominique Pelle)
diff --git a/src/testdir/test_popupwin.vim b/src/testdir/test_popupwin.vim
index 123d12a..16e4519 100644
--- a/src/testdir/test_popupwin.vim
+++ b/src/testdir/test_popupwin.vim
@@ -1857,7 +1857,7 @@
   let winid = popup_create('something', #{filter: function('MyPopupFilter', [{}])})
   call test_garbagecollect_now()
   redraw
-  " Must not crach caused by invalid memory access
+  " Must not crash caused by invalid memory access
   call feedkeys('j', 'xt')
   call assert_true(v:true)