Make object_length_linter consistent accross

[gdequeiroz]

It looks like the default argument for theobject_length_linter function is 20:
https://github.com/jimhester/lintr/blob/3312afafafd673aa7ec74231b39d83038508023c/R/object_name_linters.R#L182

but the default for calling it is 30:
https://github.com/jimhester/lintr/blob/87658ec425ca5cb2c733444ab6beef45dbb31cfe/R/zzz.R#L96

It makes sense to make them consistent, so we should change the default argument to 30.

[dragosmg]

I think this issue can be closed. Thanks @gdequeiroz for the good first issue label and thanks @jimhester for reviewing and approving my PR.