[PR][php-langspec][#157][master] - Removed Throwable Interface Method Modifiers

You can view the Pull Request on github:
https://github.com/php/php-langspec/pull/157

Pull Request Description:
> All methods declared in an interface must be implicitly or explicitly
public, and they must not be declared `abstract`.

This quote is directly from the specification itself, however, the `Throwable` interface uses the `abstract` modifier and a `public` access modifier (unlike all other interfaces in the same document). I removed both modifiers from the methods in the `Throwable` interface example to ensure consistency.

And apparently the required `function` keyword was missing too.