updates to README

Great package!!!!!

Two minor updates to the README:

(1) load the maps and raster packages.  The functions that depend on these do not load them, and throw errors.  For example, if I don't load the `raster` package and try to run `pbdb_map_richness()`, I get the following error 
```
Error in as.double(y) : 
  cannot coerce type 'S4' to vector of type 'double'
```
(2) fix typo in name of data argument for `pbdb_map_richness()`, to be consistent with other examples.