Update Label and documentation for assess_has_vignettes

[nicolekjones]

https://pharmar.github.io/riskmetric/reference/metric_score.pkg_metric_has_vignettes.html

Your documentation says that assess_has_vignettes() provides a binary indicator variable in response, but it actually tallys the number of exported namespaces

[nicolekjones]

Got my wires crossed. I realize I was looking at pkg_score() documentation