docs: clarify default value for secretsTtl

[george1410]

Adding the default value for secretsTtl to the docs. Default is 1 week per below:

fastify-jwt-jwks/index.js

Lines 196 to 197 in a4ebba7

	// Check if secrets cache is wanted - Convert milliseconds to seconds and cache for a week by default
	const ttl = parseFloat('secretsTtl' in options ? options.secretsTtl : '604800000', 10) / 1e3

closes #24

[bilalshareef]

LGTM