Remove "what" comment

We can see that the hashes are specified as strings, no need to comment
this.

Author: tcharding

src/lib.rs

@@ -201,7 +201,7 @@ impl MiniscriptKey for bitcoin::secp256k1::XOnlyPublicKey {
 }
 
 impl MiniscriptKey for String {
-    type Sha256 = String; // specify hashes as string
+    type Sha256 = String;
     type Hash256 = String;
     type Ripemd160 = String;
     type Hash160 = String;