message: Update README.md
  Use ibuf variable name in decodeIndexBuffer example
  Fixes
diff: @@ -159,7 +159,7 @@ You can then either serialize `vbuf`/`ibuf` as is, or compress them further. To
  ```c++
  int resvb = meshopt_decodeVertexBuffer(vertices, vertex_count, sizeof(Vertex), &vbuf[0], vbuf.size());
  -int resib = meshopt_decodeIndexBuffer(indices, index_count, &buffer[0], buffer.size());
  +int resib = meshopt_decodeIndexBuffer(indices, index_count, &ibuf[0], ibuf.size());
  assert(resvb == 0 && resib == 0);
  ```
  