add a good example for the approx_const lint

Author: dario23

clippy_lints/src/approx_const.rs

@@ -23,7 +23,13 @@ declare_clippy_lint! {
     ///
     /// **Example:**
     /// ```rust
+    /// // Bad
     /// let x = 3.14;
+    /// let y = 1_f64 / x;
+    ///
+    /// // Good
+    /// let x = std::f32::consts::PI;
+    /// let y = std::f64::consts::FRAC_1_PI;
     /// ```
     pub APPROX_CONSTANT,
     correctness,