doc: remove problematic example from README

Remove Buffer constructor example from security reporting examples. Even
though the example text focuses on API compatibility, the pull request
cited is about zero-filling vs. not zero-filling, which is not an API
compatibility change (or at least is not unambiguously one). The fact
that it's a pull request is also problematic, since it's not reporting a
security issue but instead proposing a way to address one that has
already been reported publicly. Finally, the text focuses on the fact
that it was not deemed worth of backporting, but that was determined by
a vote by a divided CTC. It is unreasonable to ask someone reporting an
issue to make a determination that the CTC/TSC is divided on.

In short, it's not a good example for the list it is in. Remove it.

Refs: https://github.com/nodejs/node/pull/23759#discussion_r226804801

PR-URL: https://github.com/nodejs/node/pull/23817


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>