Change constructor signature from fun() to constructor()

Now constructors are rendered as functions, i.e `fun ClassName()`, but it makes more sense to render it as a secondary constructor: `constructor()`

Look at `Constructors` tab [here](https://ignatberesnev.github.io/static/dokka/1.7.20-showcase/stdlib/kotlin-stdlib/kotlin.random/-random/index.html)

Classes to look at: `KotlinSignatureProvider`