Use `.S3method()` rather than `registerS3method()`

[fkohrt]

The cookbook currently encourages the use of registerS3method(). The documentation at help("registerS3method", "base") says, however, that it is “not intended to be called directly”. As an alternative, the method .S3method() can be used (see also Kurt Hornik: S3 Method Lookup).

https://github.com/rstudio/rmarkdown-cookbook/blob/ac81dd085eca9e71365743518d8a6a1d56430a68/examples/print-method.Rmd#L13-L16

See also #186