patch 8.2.3914: various spelling mistakes in comments

Problem:    Various spelling mistakes in comments.
Solution:   Fix the mistakes. (Dominique Pellé, closes #9416)
diff --git a/src/fold.c b/src/fold.c
index d4a6267..7d993f7 100644
--- a/src/fold.c
+++ b/src/fold.c
@@ -2513,7 +2513,7 @@
 	     */
 	    while (!got_int)
 	    {
-		// set concat to 1 if it's allowed to concatenated this fold
+		// set concat to 1 if it's allowed to concatenate this fold
 		// with a previous one that touches it.
 		if (flp->start != 0 || flp->had_end <= MAX_LEVEL)
 		    concat = 0;