Fix missing comma.

setup.py

@@ -89,7 +89,7 @@ setup(name='dulwich',
       scripts=['bin/dulwich', 'bin/dul-receive-pack', 'bin/dul-upload-pack'],
       classifiers=[
           'Development Status :: 4 - Beta',
-          'License :: OSI Approved :: Apache Software License'
+          'License :: OSI Approved :: Apache Software License',
           'Programming Language :: Python :: 2.7',
           'Programming Language :: Python :: 3.4',
           'Programming Language :: Python :: 3.5',