Update ogr_layer_algebra.py avoiding unintended misinterpretation

Else it sounds like "that town has not (even) a single taxi."

swig/python/gdal-utils/osgeo_utils/ogr_layer_algebra.py

@@ -372,7 +372,7 @@ def main(argv=sys.argv):
         cnt = input_ds.GetLayerCount()
         if cnt != 1:
             print(
-                "Input datasource has not a single layer, so you should specify its name with -input_lyr",
+                "Input datasource has more than just a single layer, so you should specify which layer with -input_lyr",
                 file=sys.stderr,
             )
             return 1