Fix incorrect sack example

[krlawrence]

As reported by @anglepei (I moved your text to here from #75)

Likewise, minus will subtract the values before putting them into the sack. Note that the values are subtracted from the sack’s initialization value.
`g.V().has('code','AUS').sack(assign).by('runways').
V().has('code','SAF').out().
sack(mult).by('runways').sack().fold()

[-5,-2,-1,-4]`

hi,this code should be wrong, it should be minus.

[krlawrence]

Included in v279-3.3.3 release. May 29, 2018