Documentation: qat_rl: make rate limiting wording clearer

Documentation: qat_rl: make rate limiting wording clearer

The term "capability" typically refers to an ability to perform an
action, whereas "capacity" denotes a measurable amount of resources.

Since the sysfs-driver-qat_rl document describes remaining resources
available to perform work, "capacity" is the more accurate term.

Replace "capability" with "capacity" in the document.

Signed-off-by: Fiona Trahe <fiona.trahe@intel.com>
Reviewed-by: Ahsan Atta <ahsan.atta@intel.com>
Signed-off-by: Giovanni Cabiddu <giovanni.cabiddu@intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>