PBRLighting.j3md: correct a typographic error

jme3-core/src/main/resources/Common/MatDefs/Light/PBRLighting.j3md

@@ -70,7 +70,7 @@ MaterialDef PBR Lighting {
 
         // Set to use TexCoord2 for the lightmap sampling
         Boolean SeparateTexCoord
-        // the light map is a gray scale ao map, on ly the r channel will be read.
+        // the light map is a grayscale ao map, only the r channel will be read.
         Boolean LightMapAsAOMap
         Boolean AoPackedInMRMap
         //shadows