patch 9.0.0963: function name does not match autocmd event name

Problem:    Function name does not match autocmd event name.
Solution:   Rename "optionsset" to "optionset". (closes #11630)
diff --git a/src/optionstr.c b/src/optionstr.c
index 084b438..53d3a1a 100644
--- a/src/optionstr.c
+++ b/src/optionstr.c
@@ -142,7 +142,7 @@
  * "newval"	the new value
  */
     void
-trigger_optionsset_string(
+trigger_optionset_string(
 	int	opt_idx,
 	int	opt_flags,
 	char_u  *oldval,
@@ -542,7 +542,7 @@
 #if defined(FEAT_EVAL)
 	// call autocommand after handling side effects
 	if (errmsg == NULL)
-	    trigger_optionsset_string(opt_idx, opt_flags,
+	    trigger_optionset_string(opt_idx, opt_flags,
 				   saved_oldval, saved_oldval_l,
 				   saved_oldval_g, saved_newval);
 	vim_free(saved_oldval);