Example code on spec website doesn't compile

[Lukas Eder]

Bug Report

Versions

• Driver: 0.9.0.M1

Current Behavior

The example code here doesn't compile:
https://r2dbc.io/spec/0.9.0.M1/spec/html/#transactions.transaction-definition.usage

Publisher<Void> insert1 = connection.createStatement("INSERT INTO books VALUES ('John Doe')").execute();

The return type of execute() is Publisher<? extends Result>

[Lukas Eder]

I realise I can just send a PR for this, so here: #209