Syntax error fix

- Folder: build\releases\Popcorn-Time\win\ already exists, line can be removed.
- Quotation marks wrong, trigger syntax error windows
- Slashes wrong, can trigger syntax error windows
- mkdir -p is linux commandline ;)
Update updater.nsi

No I was wrong... it doesn't exists yet.
Merge pull request #857 from r0gger/patch-2

Fixing slash direction in the updater NSI