Update firmware_update.md

Fix topic issue: https://forum.sparkfun.com/viewtopic.php?f=116&t=59059

Author: nseidle

docs/firmware_update.md

@@ -32,7 +32,7 @@ The firmware is shown at the top of the WiFi config page.
 
 From time to time SparkFun will release new firmware for the RTK product line to add and improve functionality. For most users, firmware can be upgraded over WiFi using the OTA method.
 
-* [OTA Method](docs\firmware_update\#updating-firmware-over-the-air): Connect over WiFi to SparkFun to download the latest firmware *over-the-air*. This can be done using the serial menu or while in WiFi AP Config Mode. Requires a local WiFi network.
+* [OTA Method](https://docs.sparkfun.com/SparkFun_RTK_Firmware/firmware_update/#updating-firmware-over-the-air): Connect over WiFi to SparkFun to download the latest firmware *over-the-air*. This can be done using the serial menu or while in WiFi AP Config Mode. Requires a local WiFi network.
 * [GUI Method](https://docs.sparkfun.com/SparkFun_RTK_Firmware/firmware_update/#updating-firmware-using-windows-gui): Use the [Windows, Linux, MacOS or Python GUI](https://github.com/sparkfun/SparkFun_RTK_Firmware_Uploader) and a USB cable. (The Python package has been tested on Raspberry Pi)
 * [SD Method](https://docs.sparkfun.com/SparkFun_RTK_Firmware/firmware_update/#updating-firmware-from-the-sd-card): Load the firmware on an SD card, then use a serial terminal with the *Firmware Upgrade* menu
 * [WiFi Method](https://docs.sparkfun.com/SparkFun_RTK_Firmware/firmware_update/#updating-firmware-from-wifi): Load the firmware over WiFi when the device is in WiFi AP Config Mode