Correct default invoice expiry

[valentinewallace]

We previously stated in the codebase that the default invoice expiry
stated in the spec is 2 hours. It's actually 1 hour.

Relevant spec: https://github.com/lightningnetwork/lightning-rfc/blob/master/11-payment-encoding.md#tagged-fields

[codecov]

Codecov Report

Merging #922 (4503ef3) into main (0b1f0a7) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #922      +/-   ##
==========================================
- Coverage   90.46%   90.45%   -0.02%     
==========================================
  Files          59       59              
  Lines       29789    29742      -47     
==========================================
- Hits        26950    26904      -46     
+ Misses       2839     2838       -1     

Impacted Files	Coverage Δ
lightning/src/ln/channelmanager.rs	83.33% <ø> (ø)
lightning-invoice/src/utils.rs	83.87% <100.00%> (+0.17%)	⬆️
lightning-block-sync/src/rest.rs	65.45% <0.00%> (-1.79%)	⬇️
lightning-block-sync/src/rpc.rs	78.37% <0.00%> (-1.11%)	⬇️
lightning-block-sync/src/poll.rs	91.66% <0.00%> (-0.38%)	⬇️
lightning-block-sync/src/lib.rs	95.18% <0.00%> (-0.20%)	⬇️
lightning-net-tokio/src/lib.rs	76.25% <0.00%> (-0.17%)	⬇️
lightning-block-sync/src/init.rs	93.56% <0.00%> (-0.15%)	⬇️
lightning-block-sync/src/http.rs	95.00% <0.00%> (-0.13%)	⬇️
lightning/src/ln/functional_tests.rs	96.84% <0.00%> (+0.01%)	⬆️

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