Changing the error message for sigma check

pycbc/distributions/sky_location.py

@@ -100,7 +100,7 @@ def __init__(self, **params):
             )
         if sigma < 0 or sigma > 2 * numpy.pi:
             raise ValueError(
-                'Sigma must be positive and smaller than 2π '
+                'Sigma must be non-negative and smaller than 2π '
                 '(preferably much smaller)'
             )
         if sigma > 0.35: