Merge pull request #581 from bsipocz/MAINT_error_message_typo

MAINT: Fix error message for SSA diameter

Author: msdemlei

pyvo/dal/ssa.py

@@ -410,7 +410,7 @@ def diameter(self, diameter):
 
         if not isinstance(diameter, Quantity):
             valerr = ValueError(
-                'Radius must be exactly one value, expressing degrees')
+                'Diameter must be exactly one value, expressing degrees')
 
             try:
                 # assume degrees