Fix typo (#18)

Author: toku-sa-n

src/lib.rs

@@ -62,7 +62,7 @@ impl<R> Volatile<R> {
     /// a reference. The only reason that we don't forbid non-reference types in the constructor
     /// functions is that the Rust compiler does not support trait bounds on generic `const`
     /// functions yet. When this becomes possible, we will release a new version of this library
-    /// with removed support for non-references. For these reasons it is not recommended to use
+    /// with removed support for non-references. For these reasons it is recommended to use
     /// the `Volatile` type only with references.
     ///
     /// ## Example