Someone like me who constantly is revising, proofing and correcting documents, brochures, programs, posters and online marketing for a living. Thought some others may find it amusing.
sorry to be a bore *sarcasm*
If we're gonna get really pedantic, and apparently we are, incline only means there is some slope. It does not imply a direction. So the shirt is grammatically correct, even if awkwardly phrased.