Update README.md Malleability definition

[bluk]

• Remove extra ' in definition of malleability

The README states:

A signature scheme is considered to produce malleable signatures if a passive attacker with knowledge of a public key A, message m, and valid signature σ' can produce a distinct σ' such that σ' is a valid signature of m with respect to A.

Haven't had my morning coffee but I think the first σ in valid signature _σ'_ can produce a distinct _σ'_ should be just σ (without the ').

[AaronFeickert]

It's not the most modern or general definition, but it works.