Fix doc resolution in rustc 1.48

[TheBlueMatt]

Not sure why this passes on CI but not locally, but this fixes cargo doc locally.

[codecov]

Codecov Report

Merging #894 (f1fa961) into main (0d75a63) will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #894      +/-   ##
==========================================
+ Coverage   90.26%   90.30%   +0.04%     
==========================================
  Files          57       57              
  Lines       29226    29226              
==========================================
+ Hits        26380    26392      +12     
+ Misses       2846     2834      -12     

Impacted Files	Coverage Δ
lightning/src/chain/mod.rs	100.00% <ø> (ø)
lightning/src/ln/functional_tests.rs	97.00% <0.00%> (+0.20%)	⬆️

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[ariard]

ACK f1fa961