updated checksig logo
<p>Development is supported by:
</p>
<p>
- <a href="https://checksig.io"><img width="180" src="https://raw.githubusercontent.com/checksig-custody/checksig-custody.github.io/master/img/logo/new_logo/png/logo-CheckSig.png"
+ <a href="https://checksig.io"><img width="180" src="https://raw.githubusercontent.com/checksig-custody/checksig-custody.github.io/master/img/logo/CheckSig-LogoColoreNero.png"
alt="CheckSig" /></a>
</p>
<p>
