DEVELOPERS: fix incorrect path to board/firefly/roc-rk3399-pc/

Commit
b86adfb89a8bd8765f1b0400e0f0b4285aea8f77 ("configs/roc_rk3399_pc: new
defconfig") introduced a new defconfig with the relevant entries in
the DEVELOPERS file, but one of these entries points to a non-existing
directory. This commit fixes that.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

DEVELOPERS

@@ -2581,7 +2581,7 @@ N:	Robert Rose <robertroyrose@gmail.com>
 F:	package/grpc/
 
 N:	Roberto Medina <robertoxmed@gmail.com>
-F:	board/firefly/roc-rk3399/
+F:	board/firefly/roc-rk3399-pc/
 F:	configs/roc_pc_rk3399_defconfig
 
 N:	Rodrigo Rebello <rprebello@gmail.com>