doc/go1.15: remove TODO in minor library changes section

The minor changes to the library section has been populated
with TODOs for individual packages using relnote in CL 235757,
and they've been resolved in the following CLs.

We will look things over as part of finishing touches on
the release notes, but this TODO is resolved for beta 1.

For #37419.

Change-Id: I942f81a957fe8df8f630b4406ca29f73602d080a
Reviewed-on: https://go-review.googlesource.com/c/go/+/237157


Reviewed-by: Alexander Rakoczy <alex@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>