Correct build instruction for experimental build

[tullo]

- What I did
Activated experimental features.

- How I did it
sed -i 's/EXPERIMEMTAL/EXPERIMENTAL/' BUILDING.md

- How to verify it
./bin/docker-app version
...
Experimental: on

- Description for the changelog
Correct build instruction for experimental build

[chris-crone]

Woops, thanks @tullo, LGTM!

[silvin-lubecki]

LGTM, thank you @tullo for spotting that!

[codecov]

Codecov Report

Merging #447 into master will decrease coverage by 0.21%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #447      +/-   ##
==========================================
- Coverage   62.68%   62.47%   -0.22%     
==========================================
  Files          49       49              
  Lines        2361     2361              
==========================================
- Hits         1480     1475       -5     
- Misses        678      682       +4     
- Partials      203      204       +1

Impacted Files	Coverage Δ
types/parameters/parameters.go	85.71% <0%> (-7.94%)	⬇️

---

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 3603491...7bb197d. Read the comment docs.