Update README.rst

Fix broken link

README.rst

@@ -7,7 +7,7 @@ Formulas to set up and configure php and various php libraries.
 .. 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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@@ -303,4 +303,4 @@ Installs the php-suhosin package.
 ``php.ng.xml``
 --------------
 
-Installs the php-xml package.
+Installs the php-xml package.