Typo introduction.md (#1744)

double unless typo, erased one

[no important files changed]

Author: Giacomo-Clerico

exercises/concept/assembly-line/.docs/introduction.md

@@ -71,7 +71,7 @@ end
 
 ## Unless statement
 
-The `unless`unless statement works very similarly to the `if` statement but it will run the code inside the `unless` statement if the condition is falsey.
+The `unless` statement works very similarly to the `if` statement but it will run the code inside the `unless` statement if the condition is falsey.
 
 ```ruby
 value = 1