Consider updating PFM references

[rootulp]

Context

ibc-apps/middleware/packet-forward-middleware/README.md

Lines 199 to 200 in 508e57d

	- <https://github.com/cosmos/ibc-go/pull/373>
	- <https://github.com/strangelove-ventures/governance/blob/master/proposals/2021-09-hub-ibc-router/README.md>

Problem

The first link is a closed PR and the second link is broken.

Proposal

The first link may still relevant be if that was the original PR that was the basis for this line of work but it seems confusing without context. The second link seems like it should be updated or removed.

Related: this repo may consider adopting markdown link check to get CI signal when a link is dead.