Fix import for Rust 1.22

Author: dhardy

src/lib.rs

@@ -810,7 +810,8 @@ pub trait FromEntropy: SeedableRng {
     /// errors, use the following code, equivalent aside from error handling:
     ///
     /// ```rust
-    /// use rand::{Error, prelude::*};
+    /// # use rand::Error;
+    /// use rand::prelude::*;
     /// use rand::entropy::EntropyRng;;
     ///
     /// # fn try_inner() -> Result<(), Error> {