Fix desktop build instructions

[andreygursky]

Fixes #2792 again after regression 3c91f90

[ara4n]

@dbkr was there a reason you removed the npm run build step?

[dbkr]

npm run build is included in the npm run dist step in the 'Building from source' bit, so you don't need to run both. Really it should be npm run build instead of npm run dist as the tarball is unnecessary. I'll make the instructions say this instead.

For future reference, please note the contributing guide at https://github.com/vector-im/riot-web/blob/master/CONTRIBUTING.rst: we need DCO signoff and PRs against the develop branch, (although for doc fixes like this one master may be acceptable).

[andreygursky]

Thanks for clarification.

The trade-off with users pulling master and developers forced to pull developer branch is not usual, thanks for pointing this out. Before committing I look at other commits and make mine according to this style. I haven't noticed other contributors signing their commits. Does it mean they could potentially every time bring the whole project down? E.g.: @aviraldg? Some commits contain only nickname: c1290fd, f10bc8e.