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/diff.c b/src/diff.c
index ece7bf5..120a544 100644
--- a/src/diff.c
+++ b/src/diff.c
@@ -419,7 +419,7 @@
 			off = 0;
 			if (last < line2)
 			{
-			    /* 2. delete at end of of diff */
+			    /* 2. delete at end of diff */
 			    dp->df_count[idx] -= last - lnum_deleted + 1;
 			    if (dp->df_next != NULL
 				    && dp->df_next->df_lnum[idx] - 1 <= line2)