Fix findByFirstnameLike @Query example in README [DATAJDBC-509]

[spring-projects-issues]

Jens Schauder opened DATAJDBC-509 and commented

There is an example that uses lastname instead of the correct firstname in the query

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Referenced from: pull request https://github.com/spring-projects/spring-data-jdbc/pull/198/files

Backported to: 1.1.6 (Moore SR6), 1.0.17 (Lovelace SR17)