Update custom-scalars.mdx

[mx-bernhard]

value should be of type unknown instead of number with [email protected]:

Type '(value: number) => number' is not assignable to type 'GraphQLScalarValueParser<number>'.
  Types of parameters 'value' and 'inputValue' are incompatible.
    Type 'unknown' is not assignable to type 'number'.ts(2322)
definition.d.ts(365, 3): The expected type comes from property 'parseValue' which is declared here on type 'Readonly<GraphQLScalarTypeConfig<number, number>>'

[apollo-cla]

@mx-bernhard: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Apollo Contributor License Agreement here: https://contribute.apollographql.com/

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