DOC: Add the fixing to_csv default encoding to whatsnew note

Author: Licht-T

doc/source/whatsnew/v0.21.0.txt

@@ -337,6 +337,7 @@ I/O
 - Bug in :meth:`DataFrame.to_csv` defaulting to 'ascii' encoding in Python 3, instead of 'utf-8' (:issue:`17097`)
 - Bug in :func:`read_stata` where value labels could not be read when using an iterator (:issue:`16923`)
 - Bug in :func:`read_html` where import check fails when run in multiple threads (:issue:`16928`)
+- Bug in :func:`to_csv` where the default encoding is not set correctly (:issue:`17097`)
 
 Plotting
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