improve description of body size in documentation

[kliegr]

Currently, the documentation states that
"The body size is a number of all possible paths in an RDF knowledge graph that conform the body of the rule." https://github.com/propi/rdfrules

but the more precise formulation would be

"the number of all distinct subject-object pairs derived from the variables in the body that also
occur in the head".

Note that the original statement does not refer to the head.