fix numpydoc: Update reference of pandas TextFileReader

This class lives at pandas.io.parsers.TextFileReader, however, there are
no public docs for it and it isn't linked on pandas doc pages either¹.

Use single backticks to disable linking while denoting that it is a class².

¹ https://pandas.pydata.org/pandas-docs/version/1.4/reference/api/pandas.read_csv.html
² https://numpydoc.readthedocs.io/en/latest/format.html#common-rest-concepts

augur/io/metadata.py

@@ -27,7 +27,7 @@ def read_metadata(metadata_file, id_columns=("strain", "name"), chunk_size=None)
 
     Returns
     -------
-    pandas.DataFrame or pandas.TextFileReader
+    pandas.DataFrame or `pandas.io.parsers.TextFileReader`
 
     Raises
     ------