Update custom-scalars.mdx

[mx-bernhard]

value should be of type unknown instead of number with graphql@16.8.1:

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>>'

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