fix: typo in e1s4

info/exercises/src/ex-04/ex/grammar.tex

@@ -10,7 +10,7 @@
     \(A_2 : R ::= \num ~R' \text{ and } R' ::= + R~ R' \mid \epsilon\)
     \item 
     \(B_1 : S ::= S(S)S \mid \epsilon \) \\ 
-    \(B_2 : R ::= RR \mid (R) \epsilon\)
+    \(B_2 : R ::= RR \mid (R) \mid \epsilon\)
   \end{enumerate}
 
   \begin{solution}