doc: fix invalid arg name hints for bugprone validation

The extra leading `=` or missing trailing `=` prevented clang-tidy's `bugprone-argument-comment` check from validating the parameter name, as it only matches comments formatted strictly as `/*arg=*/` (see https://clang.llvm.org/extra/clang-tidy/checks/bugprone/argument-comment.html).

src/wallet/test/coinselection_tests.cpp

@@ -24,7 +24,7 @@ static const int P2WPKH_OUTPUT_VSIZE = 31;
 static CoinSelectionParams init_default_params()
 {
     CoinSelectionParams dcsp{
-        /*rng_fast*/default_rand,
+        /*rng_fast=*/default_rand,
         /*change_output_size=*/P2WPKH_OUTPUT_VSIZE,
         /*change_spend_size=*/P2WPKH_INPUT_VSIZE,
         /*min_change_target=*/50'000,