Register the S3 method to the class, not the full function name

[DavisVaughan]

Closes tidymodels/tune#146

I made a typo 😭. We will need a minor recipes release for tune.

When you register a method with s3_register(), you supply it in the form:

s3_register("tune::tunable", "step_knnimpute", tunable.step_knnimpute)

I was accidentally doing:

s3_register("tune::tunable", "tunable.step_knnimpute", tunable.step_knnimpute)

It only fails in R devel because we currently explicitly export the tunable methods anyways, so they are "found" in the package environment during S3 lookup. This is what was changed in R devel, as noted by the NEWS entry: "S3 method lookup now by default skips the elements of the search path between the global and base environments." (i.e. any package env).

I think this change in S3 method lookup is good motivation to not explicitly export the tunable methods anyways, so we might consider also not exporting them.