Fix C/M/Y/K value range

[core-man]

CMYK values go from 0 to 100, rather than 0 to 1 (originally posted in GenericMappingTools/pygmt#1049 (comment)). This PR updates the documentation.

See the PR to fix it at GMT DOC GenericMappingTools/gmt#4956) and a Chinese reference (https://m.139zhuti.com/show/1nms24nu.html)

Below are two examples (one with 0.2/0.3/0.4/0.4 and the other with 20/30/40/40)

$ gmt begin cmyk pdf            
$ gmt basemap -Rg -JW15c -B                                              
$ gmt coast -G50 -S0.2/0.3/0.4/0.4
$ gmt end show        

$ gmt begin cmyk pdf              
$ gmt basemap -Rg -JW15c -B       
$ gmt coast -G50 -S20/30/40/40              
$ gmt end show