Tokens example is out-of-date with most recent SDK and upstream example

The example contract page at [`tokens.mdx`](https://github.com/stellar/stellar-docs/blob/main/docs/build/smart-contracts/example-contracts/tokens.mdx) is out-dated, and should be brought up to snuff with the current [example contract](https://github.com/stellar/soroban-examples/tree/main/token) in the soroban-examples repo.

Refs: #2691 