correct the check for number of neighbors in watts_strogatz_graph and newman_watts_strogatz_graph

previously the code

import networkx as nx
nx.watts_strogatz_graph(10,6,0.0)

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
neighbors. This patch fixes that.