documentation fix

Co-authored-by: ValuedMammal <[email protected]>

crates/chain/src/keychain.rs

@@ -49,7 +49,7 @@ impl Balance {
     }
 }
 
-/// A tuple of keychain index and corresponding [`ScriptBuf`].
+/// A tuple of keychain index and an arbitrary `T` representing the indexed value.
 pub type Indexed<T> = (u32, T);
 /// A tuple of keychain, index and the [`ScriptBuf`] derived at that location.
 pub type KeychainIndexed<K, T> = ((K, u32), T);