Use correct class ref syntax

Author: hyanwong

tsinfer/formats.py

@@ -3297,7 +3297,7 @@ def insert_proxy_samples(
         **kwargs,
     ):
         """
-        Take a set of samples from a {class}`VariantData` instance and create additional
+        Take a set of samples from a :class:`.VariantData` instance and create additional
         "proxy sample ancestors" from them, returning a new :class:`.AncestorData`
         instance including both the current ancestors and the additional ancestors
         at the appropriate time points.