update old function name in comment

src/lv_widgets/lv_textarea.c

@@ -221,7 +221,7 @@ lv_obj_t * lv_textarea_create(lv_obj_t * par, const lv_obj_t * copy)
 
 /**
  * Insert a character to the current cursor position.
- * To add a wide char, e.g. 'Á' use `lv_txt_encoded_conv_wc('Á')`
+ * To add a wide char, e.g. 'Á' use `_lv_txt_encoded_conv_wc('Á')`
  * @param ta pointer to a text area object
  * @param c a character (e.g. 'a')
  */