Clarify "special" in pbp dictionary

[mrcaseb]

closes #163

[mrcaseb]

Tests are failing because we don't have a 2023 roster file released

nflreadr::load_rosters(2023)
#> Warning in readRDS(con): cannot open URL
#> 'https://github.com/nflverse/nflverse-data/releases/download/rosters/roster_2023.rds':
#> HTTP status was '404 Not Found'
#> Warning: Failed to readRDS from
#> <https://github.com/nflverse/nflverse-data/releases/download/rosters/roster_2023.rds>
#> ── nflverse  ───────────────────────────────────────────────────────────────────
#> Null data.table (0 rows and 0 cols)

^{Created on 2023-07-14 with reprex v2.0.2}