Update busboy link pointing to nonexistent section

[krzysdz]

Busboy readme file structure changed when 1.0.0 was released (in mscdex/busboy@54a8683 to be exact).

[krzysdz]

By the way, it might be worth noting in the readme that fieldNestingDepth is a limit enforced by multer and not busboy:

multer/README.md

Line 271 in 2e2af08

An object specifying the size limits of the following optional properties. Multer passes this object into busboy directly, and the details of the properties can be found on [busboy's page](https://github.com/mscdex/busboy#busboy-methods).

It took me a moment to realise that the details of the fieldNestingDepth property can't be found on busboy's page.

[krzysdz]

Speaking of fieldNestingDepth, this seems to be incorrect:

multer/README.md

Line 318 in 2e2af08

- `fieldNestingDepth` -- set to the minimum depth your field names require (e.g. `3` for `a[b][c]`)

a[b][c] should work with fieldNestingDepth: 2 as suggested earlier in the readme.

Additionally, shouldn't it be set to the maximum depth that field names require?

[UlisesGascon]

I agree! We need to work a bit on fieldNestingDepth 👍