refactor: Slight adjustments to doc-strings

HepLean/PerturbationTheory/FieldStatistics/ExchangeSign.lean

@@ -25,7 +25,9 @@ namespace FieldStatistic
 variable {𝓕 : Type}
 
 /-- The exchange sign, `exchangeSign`, is defined as the group homomorphism
+
   `FieldStatistic →* FieldStatistic →* ℂ`,
+
   for which `exchangeSign a b` is `-1` if both `a` and `b` are `fermionic` and `1` otherwise.
   The exchange sign is the sign one picks up on exchanging an operator or field `φ₁` of statistic
   `a` with an operator or field `φ₂` of statistic `b`, i.e. `φ₁φ₂ → φ₂φ₁`.