Update REAMDE.md

Author: yisiox

README.md

@@ -22,7 +22,7 @@ This repository contains a formalization of Mazurkiewicz trace theory in the Lea
 
 ### Misc
 - `List.lean` contains misc. definitions and lemmas for `List` (string) manipulation.
-- `Occurences.lean` formalizes ordering of symbol occurrences.
+- `Occurrence.lean` formalizes ordering of symbol occurrences.
 - `EdgeSubset.lean` is a proof of the negation of a claim in The Book of Traces (Proposition 1.4.2).
 - `Computability.lean` supplies a full proof of Kleene's theorem together with mathlib4.