docs/alloy-model: add linear fee function model

In this commit, we add a model for the linear fee function we use in lnd
for fee bumping. This models can be used to reproduce the issue reported
in https://github.com/lightningnetwork/lnd/issues/8741, and can also be
shown that that bug fix resolves a counter example found by the model
checker.

docs/alloy-models/README.md

@@ -19,7 +19,7 @@ in development and understanding.
 
 Alloy is useful as when used during upfront software design (or even after the
 fact), one can create a formal model of a software system to gain better
-confidence re the _correctness_ of a system. The model can then be translated
+confidence in the _correctness_ of a system. The model can then be translated
 to code. Many times, writing the model (either before or after the code) can
 help one to better understand a given software system. Models can also be used
 to specify protocols in p2p systems such as Lightning (as it [supports temporal