gutterBreakpoints typo + gutterBreakpoints missing in @types

**Typo:**

There's a typo in the code, causing a mis-match with the README example.

the README uses `gutterBreakpoints` (lowercase p), which does not work

I tried `gutterBreakPoints` (uppercase P) and it does work. I don't know if the code is wrong or if the doc is wrong, but one of them is wrong.

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**Missing Type:**

Also, in the `@types/react-responsive-masonry"` package, the `gutterBreakpoints` prop is missing altogether.