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/vim9cmds.c b/src/vim9cmds.c
index 4e5336c..d341154 100644
--- a/src/vim9cmds.c
+++ b/src/vim9cmds.c
@@ -840,7 +840,7 @@
 
     if (cctx->ctx_skip != SKIP_YES)
     {
-	// If we know the type of "var" and it is a not a supported type we can
+	// If we know the type of "var" and it is not a supported type we can
 	// give an error now.
 	vartype = ((type_T **)stack->ga_data)[stack->ga_len - 1];
 	if (vartype->tt_type != VAR_LIST