Fixed a wrong check in watts_strogatz_graph

Dear networkx developers, 

there was a wrong check in watts_strogatz_graph which made the following valid code break:

import networkx as nx
nx.watts_strogatz_graph(10,6,0.0)

This would give the following error:

networkx.exception.NetworkXError: k>=n/2, choose smaller k or larger n

However, it should yield a ring network, with every node attached to 6
neighbours. This patch fixes it.  Could you please merge?

Best regards,
Andreas Amann