Make tile_map doctest faster by using a smaller zoom level

Just plotting the Southern Hemisphere at zoom level 1.

Author: weiji14

pygmt/datasets/tile_map.py

@@ -94,17 +94,18 @@ def load_tile_map(region, zoom="auto", source=None, lonlat=True, wait=0, max_ret
     >>> import contextily
     >>> from pygmt.datasets import load_tile_map
     >>> raster = load_tile_map(
-    ...     region=[103.60, 104.06, 1.22, 1.49],  # West, East, South, North
+    ...     region=[-180.0, 180.0, -90.0, 0.0],  # West, East, South, North
+    ...     zoom=1,  # less detailed zoom level
     ...     source=contextily.providers.Stamen.TerrainBackground,
     ...     lonlat=True,  # bounding box coordinates are longitude/latitude
     ... )
     >>> raster.sizes
-    Frozen({'band': 3, 'y': 1024, 'x': 1536})
+    Frozen({'band': 3, 'y': 256, 'x': 512})
     >>> raster.coords
     Coordinates:
       * band     (band) uint8 0 1 2
-      * y        (y) float64 1.663e+05 1.663e+05 1.663e+05 ... 1.272e+05 ...
-      * x        (x) float64 1.153e+07 1.153e+07 1.153e+07 ... 1.158e+07 ...
+      * y        (y) float64 -7.081e-10 -7.858e+04 ... -1.996e+07 -2.004e+07
+      * x        (x) float64 -2.004e+07 -1.996e+07 ... 1.996e+07 2.004e+07
     """
     # pylint: disable=too-many-locals
     if contextily is None: