Fixed a reference

Author: javiereguiluz

form/bootstrap4.rst

@@ -92,7 +92,7 @@ you'll get the error messages displayed *twice*.
 
     Since form errors are rendered *inside* the ``<label>``, you cannot use CSS
     ``:after`` to append an asterisk to the label, because it would be displayed
-    after the error message. Use the :ref:`label <reference-form-option-label-html>`
+    after the error message. Use the :ref:`label <reference-form-option-label>`
     option instead.
 
 Checkboxes and Radios