spi: Fix typo on SPI_MEM help text

The correct form is "a high-level", so fix it accordingly.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Mark Brown <broonie@kernel.org>