BIP-99: Replace placeholder with relevant link for Inflatacoin fork

[torrpriius]

This PR resolves a remaining TODO in BIP-99 by replacing the placeholder [non_proportional_inflatacoin_fork] with a direct reference to a historical Bitcointalk discussion about the proposed Inflatacoin fork.

Changes

• Replaced:
  [non_proportional_inflatacoin_fork] TODO missing link

with:

[3] Discussion on consequences of forks and schisms: https://bitcointalk.org/index.php?topic=563972.0

[jonatack]

Unsure about the proposed link. It might be more useful to either fix the footnotes formatting so that they work, or remove them.

[torrpriius]

@jonatack
I've updated the footnote formatting to match the rest of the document. The placeholder [non_proportional_inflatacoin_fork] has been replaced with a numbered reference [3], and all in-text mentions have been aligned accordingly. The link now points to a relevant Bitcointalk discussion that illustrates the concept of a "schism hardfork" described in that section.