Update license identifier

Author: jsh9

setup.cfg

@@ -8,7 +8,7 @@ url = https://github.com/jsh9/python-plot-utils
 license = GPL v3.0
 license_file = LICENSE
 classifiers =
-    License :: OSI Approved :: GPL License
+    License :: OSI Approved :: GNU General Public License v3 (GPLv3)
     Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3.4