patch 8.1.1955: tests contain typos

Problem:    Tests contain typos.
Solution:   Correct the typos. (Dominique Pelle)
diff --git a/src/testdir/screendump.vim b/src/testdir/screendump.vim
index cc1c239..d04d96d 100644
--- a/src/testdir/screendump.vim
+++ b/src/testdir/screendump.vim
@@ -24,7 +24,7 @@
   let reference = 'dumps/' . a:filename . '.dump'
   let testfile = 'failed/' . a:filename . '.dump'
 
-  " Redraw to execut the code that updates the screen.  Otherwise we get the
+  " Redraw to execute the code that updates the screen.  Otherwise we get the
   " text and attributes only from the internal buffer.
   redraw