Add demo for TF12

Author: linev

demo/examples.json

@@ -192,7 +192,8 @@
      { "name": "ff2", "file": "ff2.root", "item": "c1Dia;1" },
      { "name": "ff3", "file": "ff3.root", "item": "c1Dia;1" },
      { "name": "cb", "file": "tutorials_math.root", "item": "crystallball", "title": "Example of crystallball math function, output of tutorials/math/CrystalBall.C" },
-     { "name": "chebyshev", "file": "tutorials_math.root", "item": "chebyshev_pol", "title": "Example of Chebyshev polynoms, output of tutorials/math/chebyshev_pol.C" }
+     { "name": "chebyshev", "file": "tutorials_math.root", "item": "chebyshev_pol", "title": "Example of Chebyshev polynoms, output of tutorials/math/chebyshev_pol.C" },
+     { "name": "f12", "file": "f12.root", "item": "c1", "title" : "Example of TF12 function from documentaion" }
    ],
    "TF2" : [
      { "name": "tf2", "json": "tf2.json.gz", "opt": "", "title": "Example of TF2 drawing" },