Improve the generated template README

[macedogm]

Small suggestion to improve the generated template README with:

1. Update the example command.
2. The status referenced fixed, but the text under investigation.
3. Some formatting improvements.
4. Typo fixes.

Before

After

[cpanato]

lgtm

thanks

[cpanato]

ah sorry

i think you can keep the link

Suggested change

	"download [vexctl] and append new statements using\n" +
	"download [vexctl](https://github.com/openvex/vexctl) and append new statements using\n" +

[macedogm]

The auto generated template will be like this, because it uses the shortcut reference link.

---

OpenVEX Templates Directory

This directory contains the OpenVEX data for this repository.
The files stored in this directory are used as templates by
vexctl generate when generating VEX data for a release or
a specific artifact.

To add new statements to publish data about a vulnerability,
download vexctl and append new statements using
vexctl add. For example:

vexctl add --in-place main.openvex.json --product pkg:oci/test --vuln CVE-2014-1234567 --status under_investigation

That will add a new VEX statement expressing that the impact of
CVE-2014-1234567 is under investigation in the test image. When
cutting a new release, for pkg:oci/test the new file can be
incorporated to the release's VEX data.

Read more about OpenVEX

To know more about generating, publishing and using VEX data
in your project, please check out the vexctl repository and
documentation.

OpenVEX also has an examples repository with samples and docs.

---

We use the same shortcut in both references to vexctl, because the original target link is the same.

[cpanato]

thanks