Corrected spacing to four spaces

docs/control-structures.rst

@@ -110,9 +110,11 @@ Solidity internally allows tuple types, i.e. a list of objects of potentially di
 
     contract C {
         uint[] data;
+
         function f() returns (uint, bool, uint) {
             return (7, true, 2);
         }
+
         function g() {
             // Declares and assigns the variables. Specifying the type explicitly is not possible.
             var (x, b, y) = f();