chore: update type assertion

Author: d-yokoi

test/psbt.spec.ts

@@ -14,7 +14,7 @@ const initBuffers = (object: any): typeof preFixtures =>
     const data = result[1];
     const encoding = result[2];
 
-    return Buffer.from(data, encoding as any);
+    return Buffer.from(data, encoding as BufferEncoding);
   });
 
 const fixtures = initBuffers(preFixtures);