Update examples with hidden lines (#1476)

* Update example.rs to have correct indent

The three hidden lines in example.rs now have four spaces indent for the hidden lines.

* Update mdbook.md

guide/src/format/example.rs

@@ -1,6 +1,6 @@
 fn main() {
     println!("Hello World!");
 #
-#    // You can even hide lines! :D
+#     // You can even hide lines! :D
-#   println!("I am hidden! Expand the code snippet to see me");
+#     println!("I am hidden! Expand the code snippet to see me");
 }

guide/src/format/mdbook.md

@@ -156,7 +156,7 @@ To call the `add_one` function, we pass it an `i32` and bind the returned value
 #
 # fn add_one(num: i32) -> i32 {
 #     num + 1
-#}
+# }
 ```
 ````
 
@@ -170,7 +170,7 @@ That is, it looks like this (click the "expand" icon to see the rest of the file
 #
 # fn add_one(num: i32) -> i32 {
 #     num + 1
-#}
+# }
 ```
 
 ## Inserting runnable Rust files