Update README.rst

Fix broken link

README.rst

@@ -6,7 +6,7 @@ Formulas to update, upgrade, and dist-upgrade within apt.
 .. 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>`_.
 
 Available states
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