Merge pull request #1416 from cmu-delphi/ttownes/NCHSmortality_edits

Updated broken link to CDC epiweek

Author: nmdefries

docs/api/covidcast-signals/nchs-mortality.md

@@ -68,7 +68,7 @@ York State in our reports.
 
 We report the NCHS Mortality data in a weekly format (`time_type=week` &
 `time_value={YYYYWW}`, where `YYYYWW` refers to an epiweek). The CDC defines
-the [epiweek](https://wwwn.cdc.gov/nndss/document/MMWR_Week_overview.pdf) as
+the [epiweek](https://web.archive.org/web/20210623224758/https://wwwn.cdc.gov/nndss/document/MMWR_Week_overview.pdf) as
 seven days, from Sunday to Saturday. We check the week-ending dates provided in
 the NCHS morality data and use Python package
 [epiweeks](https://pypi.org/project/epiweeks/) to convert them into epiweek