Fix formatting in unstable book's attr-literals section

src/doc/unstable-book/src/language-features/attr-literals.md

@@ -15,16 +15,16 @@ The `attr_literals` unstable feature allows other types of literals to be used
 in attributes. Here are some examples of attributes that can now be used with
 this feature enabled:
 
-+```rust,ignore
-+#[attr]
-+#[attr(true)]
-+#[attr(ident)]
-+#[attr(ident, 100, true, "true", ident = 100, ident = "hello", ident(100))]
-+#[attr(100)]
-+#[attr(enabled = true)]
-+#[enabled(true)]
-+#[attr("hello")]
-+#[repr(C, align = 4)]
-+#[repr(C, align(4))]
-+```
+```rust,ignore
+#[attr]
+#[attr(true)]
+#[attr(ident)]
+#[attr(ident, 100, true, "true", ident = 100, ident = "hello", ident(100))]
+#[attr(100)]
+#[attr(enabled = true)]
+#[enabled(true)]
+#[attr("hello")]
+#[repr(C, align = 4)]
+#[repr(C, align(4))]
+```