Replace instances of "is supposed to" with "is intended to"

[DennisClark]

The use of the expression "is supposed to" in several places in readthedocs is odd and perhaps a bit misleading due to the usual connotations of "supposed to". For example, see
https://vulnerablecode.readthedocs.io/en/latest/tutorial_add_new_improver.html#implement-the-interesting-advisories-property

This property is supposed to return a QuerySet of Advisory...

would be much better as

This property is intended to return a QuerySet of Advisory...

It would be nice to improve all instances of this expression in readthedocs.

[johnmhoran]

I've fixed all instances of supposed to in the RTD.

[johnmhoran]

Done and closing.