Fix `Reading` examle code

[y-yagi]

The current example code has the following compile error.

error[E0616]: field `entries` of struct `atom_syndication::feed::Feed` is private
  --> src/main.rs:20:63
   |
20 |             println!("Atom feed first entry: {:?}", atom_feed.entries[0].title)
   |                                                               ^^^^^^^ private field
   |

error[E0616]: field `title` of struct `atom_syndication::entry::Entry` is private
  --> src/main.rs:20:74
   |
20 |             println!("Atom feed first entry: {:?}", atom_feed.entries[0].title)
   |                                                                          ^^^^^ private field

This patch fixed that issue and changed it to work.