design-2.0-cluster-parameters: escape final _

In rst a final _ is used for linking. Escaping it prevents an error at
compile time on design-2.0-cluster-parameters.rst

Reviewed-by: iustinp

doc/design-2.0-cluster-parameters.rst

@@ -70,7 +70,7 @@ backend parameter
 
   For example: memory, vcpus, auto_balance
 
-  All these parameters will be encoded into constants.py with the prefix "BE_"
+  All these parameters will be encoded into constants.py with the prefix "BE\_"
   and the whole list of parameters will exist in the set "BES_PARAMETERS"
 
 proper parameter