change readme link to full URL

This PR replaces the relative link with the full URL for the watermasking readme so that the link will also work in hyp3-docs, which references this readme file for the asf_tools information page.

src/asf_tools/README.md

@@ -166,5 +166,5 @@ For details on the algorithm see the `asf_tools.flood_map.make_flood_map` docstr
 
 The `asf_tools.watermasking` sub-package allows you to create a watermasking dataset 
 over an arbitrary ROI using OpenStreetMap and ESA WorldCover data.
-Note, this program requires `osmium-tool`. See [README.md](watermasking/README.md) 
-for more information on setup and usage.
+Note, this program requires `osmium-tool`. See the [watermasking subpackage readme]([watermasking/README.md](https://github.com/ASFHyP3/asf-tools/blob/main/src/asf_tools/watermasking/README.md)https://github.com/ASFHyP3/asf-tools/blob/main/src/asf_tools/watermasking/README.md) 
+for more information on setup and usage.