Use AeadCore::generate_nonce in rustdoc examples

[tarcieri]

This provides a better example closer to real-world code.

See: https://docs.rs/aead/0.5.1/aead/trait.AeadCore.html#method.generate_nonce

[codecov-commenter]

Codecov Report

Merging #466 (40b687b) into master (88a7deb) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #466   +/-   ##
=======================================
  Coverage   87.16%   87.16%           
=======================================
  Files          40       40           
  Lines        1956     1956           
=======================================
  Hits         1705     1705           
  Misses        251      251           

Impacted Files	Coverage Δ
aes-gcm-siv/src/lib.rs	81.81% <ø> (ø)
aes-gcm/src/lib.rs	86.79% <ø> (ø)
aes-siv/src/lib.rs	75.00% <ø> (ø)
ccm/src/lib.rs	96.58% <ø> (ø)
chacha20poly1305/src/lib.rs	76.92% <ø> (ø)
deoxys/src/lib.rs	86.20% <ø> (ø)
eax/src/lib.rs	90.62% <ø> (ø)
xsalsa20poly1305/src/lib.rs	85.10% <ø> (ø)

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