example value documentation lifted from `OpenAPI` doesn't make sense in `OpenRPC` context.

[aatifsyed]

spec/spec.md

Line 252 in 0e08a5c

<a name="example-value"></a>value | Any | Embedded literal example. The `value` field and `externalValue` field are mutually exclusive. To represent examples of media types that cannot naturally represented in JSON, use a string value to contain the example, escaping where necessary.

It's always going to be valid JSON.
Should also clarify that one of value or externalValue are required