Fix link to instructions

The link to the installation and usage instructions does not work. This fixes it.

README.rst

@@ -7,7 +7,7 @@ Formulas to set up and configure the Apache HTTP server.
 .. note::
 
     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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