Shapefile fetch example in README uses hard-coded filename

The following code example from the README uses a hard-coded shpz file name instead of the feature_file requested from the API with geomark.feature()

```
from geomark import Geomark

gm = Geomark('gm-abcdefghijklmnopqrstuv0bcislands')
feature_file = gm.feature('shpz')

with open('bc_islands.shpz', 'wb') as file:
    file.write(feature_file)
```