fix link to formula usage instructions

README.rst

@@ -8,7 +8,7 @@ Install nginx either by source or by package.
 
 
     See the full `Salt Formulas installation and usage instructions
-    <http://docs.saltstack.com/topics/conventions/formulas.html>`_.
+    <http://docs.saltstack.com/en/latest/topics/development/conventions/formulas.html>`_.
 
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