time.rs: remove "Basic usage text"

Only one example is given (for each method)

Author: tshepang

core/src/time.rs

@@ -617,8 +617,6 @@ impl Duration {
     ///
     /// # Examples
     ///
-    /// Basic usage:
-    ///
     /// ```
     /// use std::time::Duration;
     ///
@@ -640,8 +638,6 @@ impl Duration {
     ///
     /// # Examples
     ///
-    /// Basic usage:
-    ///
     /// ```
     /// use std::time::Duration;
     ///
@@ -700,8 +696,6 @@ impl Duration {
     ///
     /// # Examples
     ///
-    /// Basic usage:
-    ///
     /// ```
     /// use std::time::Duration;
     ///
@@ -758,8 +752,6 @@ impl Duration {
     ///
     /// # Examples
     ///
-    /// Basic usage:
-    ///
     /// ```
     /// use std::time::Duration;
     ///
@@ -814,8 +806,6 @@ impl Duration {
     ///
     /// # Examples
     ///
-    /// Basic usage:
-    ///
     /// ```
     /// use std::time::Duration;
     ///