Update README.md: Typos, Imports

[takahser]

• PR scope: fix typos, add missing imports to code example
• also, I noticed that you chose the namings KeyedTree and IndexTree and I think, IndexedTree would be more consistent here. But since this would require some refactoring, I didn't make any corresponding changes.

[frisitano]

• PR scope: fix typos, add missing imports to code example

Thank you for the contribution!

• also, I noticed that you chose the namings KeyedTree and IndexTree and I think, IndexedTree would be more consistent here. But since this would require some refactoring, I didn't make any corresponding changes.

I had initially considered IndexedTree but chose IndexTree instead. Upon reflection I think that was the wrong decision. I agree with you that IndexedTree is more consistent. I've added an issue to change this for the next version: #29 .

[frisitano]

Looks good to me, thanks for the fixes!