I prefer it as written, which is a full grammatical sentence. Dropping the "an" makes it a mild form of headlinese, which is fine since this is in fact a headline, but certainly not necessary.  (Extreme headlinese can lead to ambiguity, sometimes with grotesque results, as in the WWII-era headline "British Push Bottles Up German Rear", where by intention "push" = "sudden attack" and "bottles up" = "prevents movement of")