patch 8.1.2243: typos in comments

Problem:    Typos in comments.
Solution:   Fix the typos. (Dominique Pelle, closes #5160)  Also adjust
            formatting a bit.
diff --git a/src/undo.c b/src/undo.c
index 64fb205..85c041e 100644
--- a/src/undo.c
+++ b/src/undo.c
@@ -341,7 +341,7 @@
 }
 
 /*
- * Get the undolevle value for the current buffer.
+ * Get the undolevel value for the current buffer.
  */
     static long
 get_undolevel(void)