CONTRIBUTING.md: typo in PKG_LICENSE_FILES

Per r33861, it should be PKG_LICENSE and PKG_LICENSE_FILES used for licensing information.

Signed-off-by: Ian Leonard <antonlacon@gmail.com>

CONTRIBUTING.md

@@ -16,8 +16,8 @@ Makefile contents should contain:
     (E.g.: PKG_MAINTAINER:= Joe D. Hacker `<jdh@jdhs-email-provider.org`>)
 * A PKG_LICENSE tag declaring the main license of the package.
     (E.g.: PKG_LICENSE:=GPL-2.0+) Please use SPDX identifiers if possible (see list at the bottom).
-* An optional PKG_LICENSE_FILE including the filename of the license-file in the source-package.
-    (E.g.: PKG_LICENSE_FILE:=COPYING)
+* An optional PKG_LICENSE_FILES including the filename of the license-file in the source-package.
+    (E.g.: PKG_LICENSE_FILES:=COPYING)
 
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