docs: mtd: Update spi nor reference driver

The reference driver no longer exists since commit 50f1242c ("mtd:
fsl-quadspi: Remove the driver as it was replaced by spi-fsl-qspi.c").

Update reference to spi-fsl-qspi.c driver.
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/driver-api/mtd/spi-nor.rst

@@ -59,7 +59,7 @@ Part III - How can drivers use the framework?
 
 The main API is spi_nor_scan(). Before you call the hook, a driver should
 initialize the necessary fields for spi_nor{}. Please see
-drivers/mtd/spi-nor/spi-nor.c for detail. Please also refer to fsl-quadspi.c
+drivers/mtd/spi-nor/spi-nor.c for detail. Please also refer to spi-fsl-qspi.c
 when you want to write a new driver for a SPI NOR controller.
 Another API is spi_nor_restore(), this is used to restore the status of SPI
 flash chip such as addressing mode. Call it whenever detach the driver from