A better way of stating the claim is as Dr. Galvin A. Schmitt does on this [NASA site][1]: 

>as temperature rises, the **maximum sustainable water vapor concentration** increases by about 7% per degree Celsius  

Then, [look at a water vapor pressure table][2] 

Near 0 degrees C, the per cent increase per degree is about 8%.  

At 25 degrees C, the increase is about 6% per degree.  


  [1]: https://www.giss.nasa.gov/research/briefs/schmidt_05/
  [2]: http://intro.chem.okstate.edu/1515sp01/database/vpwater.html