author_position() typos in calculation for `Position_Normalized`

[dirtyplants]

There are a couple of typos in author_position() here:

https://github.com/jkeirstead/scholar/blob/5a5e04151ace7fe04cc903f29e50602e3b7fdf65/R/scholar.r#L252 https://github.com/jkeirstead/scholar/blob/5a5e04151ace7fe04cc903f29e50602e3b7fdf65/R/scholar.r#L255

     } else{
        pectentage <- NA
      }
    } else{
      pectentage <- NA

I think both instances of pectentage should refer to the object percentage instead.