Casper The Finality Gadget Paper : Update Checkpoints Frequency

According to the latest Casper FFG paper ;

> The genesis block is a checkpoint, and every block whose height in the block tree (or block number) is an exact multiple of 100 is also a checkpoint.

However, some discussions seem to indicate that this number is not set in stone ([1](https://ethresear.ch/t/two-speed-casper-ffg/2084)). I would suggest updating this section to a more up to date value or simply remove the specific value as this is more of an implementation detail.

The issue does not name a file or test. Read the current Casper FFG paper section and the linked discussion about checkpoint frequency, then locate the section containing the multiple-of-100 statement. Done means the documentation reflects the current value or removes the implementation-specific value.

Written by the indexing model from the issue text.