Updated Transaction::create() parameter name.

Renamed "Create a transaction"'s passed `$transactionId` parameter to `$merchantId` to remain consistent with the method as defined in the source code.

To clarify, the `Transaction` class' `create()` method accepts a merchant ID, not a transaction ID.

Author: JorgenVatle

README.md

@@ -96,7 +96,7 @@ $privateAppKey = 'your-private-key-goes-here';
                     'amount'   => $amount,      //value must be in cents 
                     'currency' => $currency     //see available formats https://github.com/paylike/currencies
                 );
-     $transaction = \Paylike\Transaction::create( $transactionId, $data );
+     $transaction = \Paylike\Transaction::create( $merchantId, $data );
 	// you will now have the transaction data in the $transaction variable.
 ```
 
@@ -107,4 +107,4 @@ $privateAppKey = 'your-private-key-goes-here';
 ```php
 	$card = \Paylike\Card::fetch( $cardId );
 	// you will now have the card data in the $card variable.
-```
+```