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/userfunc.c b/src/userfunc.c
index 8f66291..e1028e7 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -1035,7 +1035,7 @@
 
 	if (heredoc_concat_len > 0)
 	{
-	    // For a :def function "python << EOF" concatenats all the lines,
+	    // For a :def function "python << EOF" concatenates all the lines,
 	    // to be used for the instruction later.
 	    ga_concat(&heredoc_ga, theline);
 	    ga_concat(&heredoc_ga, (char_u *)"\n");