Fix warning (magrittr doc)

[lhehnke]

pipe html Rd warning: C:/Users/u126569/AppData/Local/Temp/RtmpMJBV8M/R.INSTALL309474dd4958/datenguideR/man/pipe.Rd:10: file link '%>%' in package 'magrittr' does not exist and so has been treated as a topic

Link in documentation for %>% is broken.

[IndrajeetPatil]

Here is a possible solution:
r-lib/usethis#478