improve README

Author: fblanqui

README.md

@@ -1,11 +1,11 @@
 Lambdapi library on natural numbers and polymorphic lists
 =========================================================
 
-- Set: type of set codes
-- Prop: definition of propositional logic
-- Eq: definition of Leibniz equality
-- FOL: definition of first-order logic
-- Bool: definition of booleans
-- Nat: definition and properties of natural numbers
-- List: definition and properties of polymorphic lists
-- NatBool: some boolean functions on natural numbers
+- `Set`: type of set codes
+- `Prop`: definition of propositional logic
+- `Eq`: definition of Leibniz equality
+- `FOL`: definition of first-order logic
+- `Bool`: definition of booleans
+- `Nat`: definition and properties of natural numbers
+- `List`: definition and properties of polymorphic lists
+- `NatBool`: some boolean functions on natural numbers