Fix Serializer example code on rustc pre-1.47
Fix Serializer example code on rustc pre-1.47

    error[E0277]: arrays only have std trait implementations for lengths 0..=32
      --> src/ser.rs:30:5
       |
    18 |     assert_eq!(buffer, b"---\nk: 107\n...\n---\nj: 106\nk: 107\n");
       |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::array::LengthAtMost32` is not implemented for `[u8; 33]`
       |
       = note: required because of the requirements on the impl of `std::cmp::PartialEq<&[u8; 33]>` for `std::vec::Vec<u8>`
       = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

dtolnay/serde-yaml