Update Usage example in README

[ezeev]

The example in the README contains a call to a function that is deprecated:

[mattjohnpowell]

Looking at the code, it looks like it's based off the old Solana Web3. i.e. not just a usage example update will fix it.

[jordaaash]

This warning is because of this: https://github.com/solana-labs/solana/blob/b0c6c31b71eac944585cdede48dac63db32007f6/web3.js/src/connection.ts#L3055-L3065

However, while deprecated, this API will still be supported at least until web3.js reaches 2.0, which will most likely occur from a total rewrite of the lib.

If anyone is interested in fixing this in the example code though, a PR is welcome!

[jordaaash]

Done in https://github.com/solana-labs/wallet-adapter/pull/522