tutorials/combustion/reactingFoam/ras/counterFlowFlame2D/0/N2: Correct name

Resolves bug-report http://www.openfoam.org/mantisbt/view.php?id=1706

tutorials/combustion/reactingFoam/ras/counterFlowFlame2D/0/N2

@@ -11,7 +11,7 @@ FoamFile
     format      ascii;
     class       volScalarField;
     location    "0";
-    object      O2;
+    object      N2;
 }
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //