Merge pull request #6867 from ethereum/docs-style-fix-usingfor

[DOCS] Bring Using For code examples inline with style guide

docs/contracts/using-for.rst

@@ -33,6 +33,7 @@ Let us rewrite the set example from the
 
     pragma solidity >=0.4.16 <0.7.0;
 
+
     // This is the same code as before, just without comments
     library Set {
         struct Data { mapping(uint => bool) flags; }
@@ -66,6 +67,7 @@ Let us rewrite the set example from the
         }
     }
 
+
     contract C {
         using Set for Set.Data; // this is the crucial change
         Set.Data knownValues;