Merge pull request #859 from ayurchuk/fix_code_block_style

Fix code style by removing `:fail` from lower-type-bounds.md

Author: heathermiller

_tour/lower-type-bounds.md

@@ -18,7 +18,7 @@ While [upper type bounds](upper-type-bounds.html) limit a type to a subtype of a
 
 Here is an example where this is useful:
 
-```tut:fail
+```tut
 trait Node[+B] {
   def prepend(elem: B): Unit
 }

_tour/self-types.md

@@ -24,12 +24,12 @@ trait User {
 }
 
 trait Tweeter {
-	this: User =>  // reassign this
-	def tweet(tweetText: String) = println(s"$username: $tweetText")
+  this: User =>  // reassign this
+  def tweet(tweetText: String) = println(s"$username: $tweetText")
 }
 
 class VerifiedTweeter(val username_ : String) extends Tweeter with User {  // We mixin User because Tweeter required it
-	def username = s"real $username_"
+  def username = s"real $username_"
 }
 
 val realBeyoncé = new VerifiedTweeter("Beyoncé")