CONTRACTS: fix doxygen

Author: Remi Delmas

src/goto-instrument/contracts/doc/developer/contracts-dev-spec-codegen.md

@@ -140,4 +140,4 @@ void contract::foo::frees(
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- @ref contracts-dev-spec-transfo-params | @ref contracts-dev-spec-spec-rewriting
+ @ref contracts-dev-spec-transform-params | @ref contracts-dev-spec-spec-rewriting