Updated SPFx On Premises notice

[KoenZomers]

Q	A
Bug fix?
New feature?
New sample?
Documentatiion fix	x
Related issues?	N/A

What's in this Pull Request?

Updated that only SharePoint 2016 uses 1.1.0 of SPFx and SharePoint 2019 should be fine to use.

[codecov-io]

Codecov Report

Merging #418 into dev will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #418   +/-   ##
=======================================
  Coverage   73.31%   73.31%           
=======================================
  Files          18       18           
  Lines         802      802           
  Branches      164      164           
=======================================
  Hits          588      588           
  Misses        164      164           
  Partials       50       50

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 53ac7a8...7913b6e. Read the comment docs.

[AJIXuMuK]

Thank you @KoenZomers for the change!