fix introduction.md in 'assembly-line' exercise (#1713)

Author: takaho9

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

@@ -88,8 +88,8 @@ end
 
 ## Else statement
 
-The `else` statement can be used in conjunction with the `if` and `unless` statements.
-The `else` statement will be executed if the `if` branch or the `unless` branch is not executed.
+The `else` statement can be used in conjunction with the `if` statement.
+The `else` statement will be executed if the `if` branch is not executed.
 
 ```ruby
 value = 1
@@ -99,11 +99,6 @@ else
   "1 is not equal to 1"
 end
 # => "1 is equal to 1"
-
-unless value < 2
-  "1 is not greater than 2"
-end
-# => "1 is greater than 2"
 ```
 
 ## "Cascading-if" statements