Replace shp by gpkg in docs

Author: juliohm

docs/src/data.md

@@ -108,7 +108,7 @@ GeoIO.formats
 ```@example data
 using GeoIO
 
-zone = GeoIO.load("data/zone.shp")
+zone = GeoIO.load("data/zone.gpkg")
 ```
 
 ### Artifacts

docs/src/data/zone.dbf

@@ -68,7 +68,7 @@ load any file with geospatial data into well-tested geometries from the
 ```@example quickstart
 using GeoIO
 
-zone = GeoIO.load("data/zone.shp")
+zone = GeoIO.load("data/zone.gpkg")
 ```
 
 Various functions are defined over these geometries, for instance: