Fix Tempo Javadoc example to use contract address instead of "USDC"

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: raubrey2014

src/main/java/com/stripe/mpp/methods/tempo/Tempo.java

@@ -13,7 +13,7 @@
  * VerifyResult result = server.charge(
  *     request.getHeader("Authorization"),
  *     Tempo.chargeIntent(),
- *     "10.000000", "USDC", "0xRecipient"
+ *     "10.000000", TempoDefaults.MAINNET_USDC, "0xRecipient"
  * );
  * }</pre>
  */