Add imports to doctests

Author: korrat

clippy_lints/src/zero_sized_map_values.rs

@@ -22,12 +22,14 @@ declare_clippy_lint! {
     /// **Example:**
     ///
     /// ```rust
+    /// # use std::collections::HashMap;
     /// fn unique_words(text: &str) -> HashMap<&str, ()> {
     ///     todo!();
     /// }
     /// ```
     /// Use instead:
     /// ```rust
+    /// # use std::collections::HashMap;
     /// fn unique_words(text: &str) -> HashSet<&str> {
     ///     todo!();
     /// }