Set preventBodyShift to true in the examples

[papb]

Running the examples in https://alertifyjs.com/ cause the body to shift because the vertical scrollbar disappears. I was about to open an issue about this but I saw #167. So there is already a way to do it, which seems to be the preventBodyShift option.

This issue is to suggest that preventBodyShift is set on the examples in https://alertifyjs.com/ because it is a bit discouraging to see a body shift like this in the first example.

[MohammadYounes]

The docs uses the defaults, I think it will be okay to set preventBodyShift in home page.

[papb]

@MohammadYounes thanks! By the way can you give an example of situation in which the preventBodyShift option would be undesirable?

[MohammadYounes]

@papb It's a developer preference. Some may go for padding the body to accommodate the missing scrollbar instead of having a disabled one.