Clarify description of get_node_secret() method

[danielgranhao]

I was confused by the original description, so I'm proposing a small change.
Probably for most, it was obvious that the secret key is not the node_id itself, but when I read this initially, I even thought that maybe the node_id/public key was being held in a SecretKey struct.

[codecov-commenter]

Codecov Report

Merging #1556 (526d381) into main (16115cd) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1556      +/-   ##
==========================================
+ Coverage   90.94%   90.97%   +0.02%     
==========================================
  Files          80       80              
  Lines       43610    43831     +221     
  Branches    43610    43831     +221     
==========================================
+ Hits        39661    39874     +213     
- Misses       3949     3957       +8     

Impacted Files	Coverage Δ
lightning/src/chain/keysinterface.rs	93.59% <ø> (ø)
lightning/src/chain/onchaintx.rs	93.98% <0.00%> (-0.93%)	⬇️
lightning/src/ln/functional_tests.rs	96.89% <0.00%> (-0.05%)	⬇️
lightning-invoice/src/lib.rs	87.39% <0.00%> (ø)
lightning/src/ln/functional_test_utils.rs	95.23% <0.00%> (ø)
lightning/src/routing/router.rs	92.57% <0.00%> (+<0.01%)	⬆️
lightning/src/ln/channelmanager.rs	84.39% <0.00%> (+<0.01%)	⬆️
lightning/src/ln/payment_tests.rs	99.25% <0.00%> (+<0.01%)	⬆️
lightning/src/ln/channel.rs	88.72% <0.00%> (+0.13%)	⬆️
lightning/src/ln/onion_route_tests.rs	97.68% <0.00%> (+0.16%)	⬆️
... and 2 more

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