Made spelling corrections, the spelling mistakes picked up by a script

written by Neil Salter.

script

include/osg/Referenced

@@ -21,8 +21,8 @@ class SG_EXPORT Referenced
         inline void ref() const { ++_refCount; }
         /** decrement the reference count by one, indicating that 
             a pointer to this object is referencing it.  If the
-            refence count goes to zero, it is assumed that this object
-            is nolonger referenced and is automatically deleted.*/
+            reference count goes to zero, it is assumed that this object
+            is no longer referenced and is automatically deleted.*/
         inline void unref() const { --_refCount; if (_refCount<=0) delete this; }
         /** return the number pointers currently referencing this object. */
         inline const int referenceCount() const { return _refCount; }