Add 'repository' to lightning-invoice's Cargo.toml

[justinmoon]

The repository doesn't show up anywhere on docs.rs or crates.io. Took me a couple minutes to find it.

[codecov-commenter]

Codecov Report

Merging #1511 (7b8f684) into main (0b77008) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1511      +/-   ##
==========================================
- Coverage   91.01%   90.97%   -0.04%     
==========================================
  Files          80       80              
  Lines       43409    43409              
  Branches    43409    43409              
==========================================
- Hits        39509    39493      -16     
- Misses       3900     3916      +16     

Impacted Files	Coverage Δ
lightning/src/util/events.rs	41.66% <0.00%> (-0.33%)	⬇️
lightning/src/ln/functional_tests.rs	96.97% <0.00%> (-0.23%)	⬇️

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