update runoff documentation

in this section of the calculations page: https://doi-usgs.github.io/hyswap/meta/calculations.html#workflow-for-determing-weighting-of-streamgages-for-area-based-runoff-calculations

• fix md format for URL links
• if possible, link directly to the data sets used (this might be better in the geospatial-data-assembly repo long-term, but for now this seems like the simplest place to add details)