Rollup merge of rust-lang#34462 - dns2utf8:leading_zeros, r=GuillaumeGomez

Add example with leading zeros

I was searching for this format very long. So I added an example to the prominent section.

I was thinking of putting the keyword leading in the corresponding section as well, what do you think?

r? @steveklabnik

Author: GuillaumeGomez

src/libcollections/fmt.rs

@@ -28,6 +28,7 @@
 //! format!("{:?}", (3, 4));          // => "(3, 4)"
 //! format!("{value}", value=4);      // => "4"
 //! format!("{} {}", 1, 2);           // => "1 2"
+//! format!("{:04}", 42);             // => "0042" with leading zeros
 //! ```
 //!
 //! From these, you can see that the first argument is a format string. It is