Average shortest path formulas

[filmenczer]

In Eq. 2.2, 2.3, and 2.5 the sum over the pairs of nodes is supposed to include each pair (i,j) once or both (i,j) and (j,i) depending on whether the network is directed or undirected. This is mentioned in the text but it may be unclear and confusing. We could rewrite the formulas so that the notation $\sum_{i,j}$ always means the same thing.

Thanks to @giaruffo for flagging this issue!

[filmenczer]

The following edits may add clarity:

• Eqs. 2.2 and 2.5: $\sum_{i,j}$ -> $\sum_{i < j}$
• Eq. 2.3: $\sum_{i,j}$ -> $\sum_{i \ne j}$