networks.scrbl: Add an explicit tag to Basic definitions.

Otherwise there is a conflict with a section with the same name in
rs.scrbl.

scribblings/networks.scrbl

@@ -10,7 +10,7 @@ is a set of variables which are updated according to their corresponding update
 functions.  The variables to be updated at each step are given by the mode.
 This model can generalise Boolean networks, TBANs, multivalued networks, etc.
 
-@section{Basic definitions}
+@section[#:tag "networks-basics"]{Basic definitions}
 
 @section{Syntactic description of networks}