Merge pull request #913 from cmu-delphi/survey/weights-manual

Point to new weights user guide

Author: krivard

docs/symptom-survey/weights.md

@@ -12,9 +12,10 @@ by Facebook. These weights are also used to produce our
 [public contingency tables](./contingency-tables.md) and the geographic aggregates
 [in the COVIDcast Epidata API](../api/covidcast-signals/fb-survey.md).
 
-Facebook has provided documentation to describe the calculation and usage of
-these weights, [available here](symptom-survey-weights.pdf). This documentation
-explains the weight methodology, gives examples of how to use the weights when
-calculating estimates, and states the known limitations of the weights. We also
-have separate information about the [survey's limitations](limitations.md) that
-affect what conclusions can be drawn from the survey data.
+Facebook has developed a [User Guide for the CTIS
+Weights](https://dataforgood.facebook.com/dfg/resources/user-guide-for-ctis-weights)
+(updated May 2022). This manual explains the weight methodology, gives examples
+of how to use the weights when calculating estimates, and states the known
+limitations of the weights. We also have separate information about the
+[survey's limitations](limitations.md) that affect what conclusions can be drawn
+from the survey data.