rackspace: Fix typos and improve 'additional_rackspace_flags' help doc
rackspace: Fix typos and improve 'additional_rackspace_flags' help doc

Signed-off-by: Carlos L. Torres <3d3f59b89fa91d9a5a81b5bf5b49df40b118dedd@rackspace.com>

meteorfox/PerfKitBenchmarker,AdamIsrael/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,meteorfox/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,AdamIsrael/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker