zstd: fix legal info

zstd is dual licensed under BSD-3-Clause or GPL-2.0 as per README.md
and source files license header.

Cc: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Rahul Bedarkar <rahulbedarkar89@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit 9b2f6548f8d2cad52fd3a5f81febf4818ee66304)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/zstd/zstd.mk

@@ -6,7 +6,7 @@
 
 ZSTD_VERSION = v1.3.3
 ZSTD_SITE = $(call github,facebook,zstd,$(ZSTD_VERSION))
-ZSTD_LICENSE = BSD-3-Clause, GPL-2.0
+ZSTD_LICENSE = BSD-3-Clause or GPL-2.0
 ZSTD_LICENSE_FILES = LICENSE COPYING
 
 ifeq ($(BR2_TOOLCHAIN_HAS_THREADS),y)