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/option.c b/src/option.c
index b76cbb0..816837d 100644
--- a/src/option.c
+++ b/src/option.c
@@ -1618,7 +1618,7 @@
 
 #if defined(FEAT_EVAL)
     if (*errmsg == NULL)
-	trigger_optionsset_string(opt_idx, opt_flags, saved_origval,
+	trigger_optionset_string(opt_idx, opt_flags, saved_origval,
 			       saved_origval_l, saved_origval_g, saved_newval);
     vim_free(saved_origval);
     vim_free(saved_origval_l);