minor change to correct case of payment details

Author: rkewlani

docs/checkout-api.html

@@ -83,7 +83,7 @@ <h2><a id="payments_0"></a>payments</h2>
 PaymentsResponse paymentResponse = checkout.payments(paymentsRequest);
 </code></pre>
 
-        <h2><a id="paymentsDetails_37"></a>payments/Details</h2>
+        <h2><a id="paymentsDetails_37"></a>payments/details</h2>
         <p>Submits details for a payment created using <strong>payments( )</strong>. This step is only needed when no
             final state has been reached on the <strong>payments( )</strong> request (for example for 3D Secure, or when
             getting redirected back directly from a payment method using an app switch).</p>