[Bug-gsl] Glitch in histogram documentation

[Kevin M. Huffenberger]

In gsl-ref_21.html#SEC339, the diagram for the histogram struct is 
incorrect:

     [ bin[0] )[ bin[1] )[ bin[2] )[ bin[3] )[ bin[5] )
  ---|---------|---------|---------|---------|---------|---  x
   r[0]      r[1]      r[2]      r[3]      r[4]      r[5]

Note bin[4] has been skipped should replace bin[5] in the diagram.

Best wishes,

Kevin