doc: board: ti: k3: Update TI firmware repository URL to GitHub

Update the TI firmware repository URL from git.ti.com to the
GitHub mirror at github.com/TexasInstruments/ti-linux-firmware
which is much more reliable.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Reported-by: Tom Rini <trini@konsulko.com>
Tested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

doc/board/ti/k3.rst

@@ -184,7 +184,7 @@ online
 
 * **TI Firmware (TIFS, DM, SYSFW)**
 
-  | **source:** https://git.ti.com/git/processor-firmware/ti-linux-firmware.git
+  | **source:** https://github.com/TexasInstruments/ti-linux-firmware
   | **branch:** ti-linux-firmware
 
 .. note::