Inconsistency on `engine_exchangeCapabilities` examples

[marioevz]

In specification section of the engine_exchangeCapabilities method description, there is an error on the examples provided where the "params" values do not take into account the external array that comprises the JSON-RPC specification.

Currently the examples state:

params: ["engine_newPayloadV1", "engine_newPayloadV2", ...]

But since the params are comprised of a single "Array of string" as parameter 1, the actual example should be:

params: [["engine_newPayloadV1", "engine_newPayloadV2", ...]]

Note the outer array representing the "params" array of the JSON-RPC format and the inner array representing the parameter 1 "Array of string".

"response" examples are correctly laid out as a single array.