security issue: typo in noopener on link rel

[dsebastien]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[x] Bug report  
[ ] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/NationalBankBelgium/stark/blob/master/CONTRIBUTING.md#got-a-question-or-problem

Current behavior

The following lines have a security bug:

• stark/showcase/src/index.html

  Line 52 in 60ea76a

  <a href="https://browsehappy.com" target="_blank" rel="noreferrer noopenenr"> upgrade your browser </a> to improve your

• stark/starter/src/index.html

  Line 49 in 77c847a

  <a href="https://browsehappy.com" target="_blank" rel="noreferrer noopenenr"> upgrade your browser </a> to improve your

Expected behavior

The rel's value should be noreferrer noopener.

Reference: https://web.dev/external-anchors-use-rel-noopener/