Fix switch firmware bash code (#27)

Signed-off-by: Anthony Floeder <anthony.floeder@hpe.com>

Author: ajfloeder

docs/guides/firmware-upgrade/readme.md

@@ -17,10 +17,9 @@ In order to upgrade the firmware on the PCIe switch, the `switchtec` kernel driv
 ```bash
 IMAGE=$1 # Provide the path to the firmware image file
 SWITCHES=("/dev/switchtec0" "/dev/switchtec1")
-for SWITCH in $SWITCHES; do switchtec fw-update $SWITCH $IMAGE --yes; done
+for SWITCH in "${SWITCHES[@]}"; do switchtec fw-update "$SWITCH" "$IMAGE" --yes; done
 ```
 
-
 ## NVMe Drive Firmware Upgrade
 
 In order to upgrade the firmware on NVMe drives attached to Rabbit, the `switchtec` and `switchtec-nvme` executables must be installed. All firmware downloads to drives are sent to the physical function of the drive which is accessible only using the `switchtec-nvme` executable.