Correct the modal close examples

[Riddler84]

Both close examples toggle only the default (first) modal, because they have the same ID.

[floriandotpy]

Github somehow messed up when merging this automatically. It's now in the docs, though: https://github.com/uikit/uikit-site/commit/9859a77f2951c0b463e86c949f1bf6c57053f7e3

Thanks!