patch 8.0.0164: outdated and misplaced comments

Problem:    Outdated and misplaced comments.
Solution:   Fix the comments.
diff --git a/src/charset.c b/src/charset.c
index 134cd09..c3b6259 100644
--- a/src/charset.c
+++ b/src/charset.c
@@ -887,7 +887,7 @@
 
 /*
  * return TRUE if 'c' is a keyword character: Letters and characters from
- * 'iskeyword' option for current buffer.
+ * 'iskeyword' option for the current buffer.
  * For multi-byte characters mb_get_class() is used (builtin rules).
  */
     int