fix rust-lang#1608: out-of-bound indexing is a runtime error (not a c…

…ompile-time error)

src/primitives/array.md

@@ -64,7 +64,7 @@ fn main() {
         }
     }
 
-    // Out of bound indexing causes compile error
+    // Out of bound indexing causes runtime error
     //println!("{}", xs[5]);
 }
 ```