correct Six String Proof diff, fixes #403

Author: noahm

src/songs/a20plus.json

@@ -28471,7 +28471,7 @@
       "charts": [
         { "lvl": 2, "style": "single", "diffClass": "beginner" },
         { "lvl": 7, "style": "single", "diffClass": "basic" },
-        { "lvl": 11, "style": "single", "diffClass": "difficult" },
+        { "lvl": 12, "style": "single", "diffClass": "difficult" },
         { "lvl": 14, "style": "single", "diffClass": "expert" },
         { "lvl": 7, "style": "double", "diffClass": "basic" },
         { "lvl": 11, "style": "double", "diffClass": "difficult" },