Update code examples to match style guide

Remove visibility

docs/control-structures.rst

@@ -271,6 +271,7 @@ because only a reference and not a copy is passed.
 
     pragma solidity >=0.4.16 <0.7.0;
 
+
     contract C {
         uint[20] x;
 
@@ -354,7 +355,7 @@ In any case, you will get a warning about the outer variable being shadowed.
     for the entire function, regardless where it was declared. The following example shows a code snippet that used
     to compile but leads to an error starting from version 0.5.0.
 
- ::
+::
 
     pragma solidity >=0.5.0 <0.7.0;
     // This will not compile