Use 0.4.21 pragma for documentation that uses "emit".

docs/introduction-to-smart-contracts.rst

@@ -80,7 +80,7 @@ registering with username and password - all you need is an Ethereum keypair.
 
 ::
 
-    pragma solidity ^0.4.20; // should actually be 0.4.21
+    pragma solidity ^0.4.21;
 
     contract Coin {
         // The keyword "public" makes those variables

docs/solidity-by-example.rst

@@ -219,7 +219,7 @@ activate themselves.
 
 ::
 
-    pragma solidity ^0.4.20; // should actually be 0.4.21
+    pragma solidity ^0.4.21;
 
     contract SimpleAuction {
         // Parameters of the auction. Times are either
@@ -376,7 +376,7 @@ high or low invalid bids.
 
 ::
 
-    pragma solidity ^0.4.20; // should actually be 0.4.21
+    pragma solidity ^0.4.21;
 
     contract BlindAuction {
         struct Bid {
@@ -529,7 +529,7 @@ Safe Remote Purchase
 
 ::
 
-    pragma solidity ^0.4.20; // should actually be 0.4.21
+    pragma solidity ^0.4.21;
 
     contract Purchase {
         uint public value;

docs/structure-of-a-contract.rst

@@ -86,7 +86,7 @@ Events are convenience interfaces with the EVM logging facilities.
 
 ::
 
-    pragma solidity ^0.4.20; // should actually be 0.4.21
+    pragma solidity ^0.4.21;
 
     contract SimpleAuction {
         event HighestBidIncreased(address bidder, uint amount); // Event

docs/types.rst

@@ -470,7 +470,7 @@ Example that shows how to use internal function types::
 
 Another example that uses external function types::
 
-    pragma solidity ^0.4.20; // should actually be 0.4.21
+    pragma solidity ^0.4.21;
 
     contract Oracle {
       struct Request {