Correct license classifier (#2)

Author: jorwoods

setup.cfg

@@ -11,7 +11,7 @@ project_urls =
     Bug Tracker = https://github.com/jorwoods/argsubparse/issues
 classifiers =
     Programming Language :: Python :: 3
-    License :: OSI Approved :: GNU GPL-3.0
+    License :: OSI Approved :: GNU General Public License v3 (GPLv3)
     Operating System :: OS Independent
 
 [options]