callr update from CRAN

[gaborcsardi]

Version: 3.7.3
Check: Rd \usage sections
Result: NOTE 
  Documented arguments not in \usage in Rd file 'get_result.Rd':
    ‘out’
  
  Functions with \usage entries need to have the appropriate \alias
  entries, and all their arguments documented.
  The \usage entries must correspond to syntactically valid R code.
  See chapter ‘Writing R documentation files’ in the ‘Writing R
  Extensions’ manual.

[gaborcsardi]

Closed by #272.