Replace "pool_options&" parameter by "const pool_options&"

[Dani-Hub]

Presumably near to non-editorial, but I think it's clear from the context that the member declaration in the class synchronized_pool_resource synopsis [memory.resource.pool.overview]:

explicit synchronized_pool_resource(pool_options& opts)
: synchronized_pool_resource(opts, get_default_resource()) { }

intended to use const pool_options& instead:

explicit synchronized_pool_resource(const pool_options& opts)
: synchronized_pool_resource(opts, get_default_resource()) { }