fixed CheckSig description

Author: fametrano

index.html

@@ -50,7 +50,7 @@
 "https://www.confluence.com/">Confluence</a> (formerly StatPro), a
 leading international risk-management provider, where the QuantLib
 project was born.  <a href="https://www.checksig.com/">CheckSig</a>, a
-crypto custodian and service provider, is sponsoring the hosting of
+crypto asset service provider, is sponsoring the hosting of
 this site.</p>
 
 <p>