_content/doc: mention fix of Convert/ConvertibleTo in 1.17
_content/doc: mention fix of Convert/ConvertibleTo in 1.17

CL 309729 fixed a bug in Convert/ConvertibleTo that we should
mention in the release notes for 1.17.

Update golang/go#47785

Change-Id: I4490c15ddf1f52404f9def19e2c9dafac754fd95
Reviewed-on: https://go-review.googlesource.com/c/website/+/343690
Trust: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website