Update intro.html

Changed "DarkSend" to "Darksend", "RC1" to "RC3", and other minor grammatical fixes.
- released v0.9.11.6 / v0.10.11.6
Merge pull request #14 from nightlydarkcoin/0.x.11.6

- released v0.9.11.6 / v0.10.11.6
Merge pull request #13 from raze182/patch-5

Update intro.html