Fix doctest on pygmt.datasets.tile_map.load_tile_map in ci_tests_legacy (#3642)

Use rio.crs.to_string() to get the 'EPSG:3857' string representation that should make the test compatible with rasterio 1.4.2 or older.

---------

Co-authored-by: Dongdong Tian <[email protected]>

Author: weiji14

pygmt/datasets/tile_map.py

@@ -125,8 +125,8 @@ def load_tile_map(
         spatial_ref  int... 0
     >>> # CRS is set only if rioxarray is available
     >>> if hasattr(raster, "rio"):
-    ...     raster.rio.crs
-    CRS.from_wkt(...)
+    ...     raster.rio.crs.to_string()
+    'EPSG:3857'
     """
     if not _HAS_CONTEXTILY:
         msg = (