Update broken link

[iampi31415]

As discussed here the readme in the rust-embedded organisation page says:

We are an official working group of the Rust language.

The link points to a 404 and there isn't any info about embedded-rust in the rust-lang website (as in the archives or governance pages.)

cc: @BartMassey

[Emilgardis]

correct would be https://rust-lang.org/governance/teams/launching-pad/#team-wg-embedded

[BartMassey]

@Emilgardis thanks! PR would be welcome, else I'll try to get to this in a bit.