Remove double `/` from GH package refs

[gaborcsardi]

E.g.

pak::pak("apache/arrow-adbc//r/adbcpostgresql")
#> ! Using bundled GitHub PAT. Please add your own PAT using `gitcreds::gitcreds_set()`.
#> Error: ! error in pak subprocess
#> Caused by error: 
#> ! Could not solve package dependencies:
#> * apache/arrow-adbc//r/adbcpostgresql: ! pkgdepends resolution error for apache/arrow-adbc//r/adbcpostgresql.
#> Caused by error: 
#> ! Can't find R package in GitHub repo apache/arrow-adbc in directory '/r/adbcpostgresql'

CC @krlmlr