patch 8.1.0528: various typos in comments

Problem:    Various typos in comments.
Solution:   Fix the typos.
diff --git a/src/fileio.c b/src/fileio.c
index afc5157..bf46522 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -8086,7 +8086,7 @@
 
 /*
  * Add an autocmd group name.
- * Return it's ID.  Returns AUGROUP_ERROR (< 0) for error.
+ * Return its ID.  Returns AUGROUP_ERROR (< 0) for error.
  */
     static int
 au_new_group(char_u *name)
@@ -8153,7 +8153,7 @@
 
 /*
  * Find the ID of an autocmd group name.
- * Return it's ID.  Returns AUGROUP_ERROR (< 0) for error.
+ * Return its ID.  Returns AUGROUP_ERROR (< 0) for error.
  */
     static int
 au_find_group(char_u *name)