learn/execution resolver function - should mention signature is specific to js server

[tonyhallett]

From https://github.com/graphql/graphql.github.io/blob/source/src/content/learn/Learn-Execution.md

A resolver function receives four arguments

I suggest that this is changed to "This resolver function receives four arguments"

Given that the specifications specifies

ResolveFieldValue(objectType, objectValue, fieldName, argumentValues)
Let resolver be the internal function provided by objectType for determining the resolved value of a field named fieldName.
Return the result of calling resolver, providing objectValue and argumentValues.

So although the resolver function with 4 arguments is specification-compliant a resolver function does not receive four arguments. Only the first two.