Update amount of data stored at IA

[vmbrasseur]

Summary.md says there are 10PB at IA. That's out of date by at least a couple of petabytes.

[jjjake]

A new IA onesheet summary was released recently: https://archive.org/download/onesheet/ia-onesheet_20160126.pdf

Future summaries like this will also be available here: https://archive.org/details/onesheet

IA is currently at 25PB of unique data. There's also a bunch of other stats on that sheet that might be interesting to add to summary.md.