std::time : fix doc variable name

Author: jersou

library/std/src/time.rs

@@ -95,8 +95,8 @@ pub use core::time::FromFloatSecsError;
 /// use std::time::{Instant, Duration};
 ///
 /// let now = Instant::now();
-/// let max_nanoseconds = u64::MAX / 1_000_000_000;
-/// let duration = Duration::new(max_nanoseconds, 0);
+/// let max_seconds = u64::MAX / 1_000_000_000;
+/// let duration = Duration::new(max_seconds, 0);
 /// println!("{:?}", now + duration);
 /// ```
 ///