fix a typo in mcomp-lang grammar

Author: lillo

mcomp-lang/mcomp-parsing/README.md

@@ -76,7 +76,7 @@ As usual in EBNF notation, the operator `*` means zero or more occurrences, `+`
 
     BasicType ::= "int" | "char" | "void" | "bool"  
 
-    Stmt ::= "return" Expr? ";" | Expr ";" | Block | "while" "(" Expr ")" Stmt 
+    Stmt ::= "return" Expr? ";" | Expr? ";" | Block | "while" "(" Expr ")" Stmt 
            | "if" "(" Expr ")" Stmt "else" Stmt  | "if" "(" Expr ")" Stmt
            | "for" "(" Expr? ";" Expr? ";" Expr? ")" Stmt