package/python-tftpy: fix legal info

Fix the following build failure raised since the addition of the package
in commit 8dd5390be6b210eaac1bf02af2fea84d4c87dcd1:

>>> python-tftpy 0.8.2 Collecting legal info
ERROR: No hash found for LICENSE.txt

Fixes:
 - http://autobuild.buildroot.org/results/9e12e0cd81bdf1a09884a340ca809c194e4dba9b

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/python-tftpy/python-tftpy.mk

@@ -7,7 +7,7 @@
 PYTHON_TFTPY_VERSION = 0.8.2
 PYTHON_TFTPY_SITE = $(call github,msoulier,tftpy,$(PYTHON_TFTPY_VERSION))
 PYTHON_TFTPY_LICENSE = MIT
-PYTHON_TFTPY_LICENSE_FILES = LICENSE.txt
+PYTHON_TFTPY_LICENSE_FILES = LICENSE
 PYTHON_TFTPY_SETUP_TYPE = setuptools
 
 $(eval $(python-package))