Fix source links in pkgdown documentation

By providing the `repo` block with a link pointing to
R-packages/covidcast/, pkgdown can build the correct URLs.

Author: capnrefsmmat

R-packages/covidcast/_pkgdown.yml

@@ -19,6 +19,13 @@ articles:
       - correlation-utils
       - multi-signals
 
+repo:
+  url:
+    home: https://github.com/cmu-delphi/covidcast/tree/main/R-packages/covidcast
+    source: https://github.com/cmu-delphi/covidcast/blob/main/R-packages/covidcast/
+    issue: https://github.com/cmu-delphi/covidcast/issues
+    user: https://github.com/
+
 reference:
   - title: Fetch data
     desc: Fetch signals and metadata from the COVIDcast API