[RPC][Trivial] Tweak exportzerocoins example to be sensible

[CaveSpectre11]

Problem

exportzerocoins help gives an example with a denomination of 5; which doesn't make any sense.

Examples:
> veil-cli exportzerocoins false 5
> curl --user myusername --data-binary '{"jsonrpc": "1.0", "id":"curltest", "method": "exportzerocoins", "params": [false 5] }' -H 'content-type: text/plain;' http://127.0.0.1:58812/

Solution

Tweak it to show an example of 10

Examples:
> veil-cli exportzerocoins false 10
> curl --user myusername --data-binary '{"jsonrpc": "1.0", "id":"curltest", "method": "exportzerocoins", "params": [false 10] }' -H 'content-type: text/plain;' http://127.0.0.1:58812/

[codeofalltrades]

utACK 3cf9ddf