Update README.md

Author: ws-garcia

README.md

@@ -31,7 +31,7 @@ The module can evaluate mathematical expressions such as:
 Allowed expressions must follow the following grammar:
 
 ```
-Expression    =     [{"("}]  SubExpr [{Operator [{"("}] SubExpr [{")"}]}] [{")"}] | {["("] ["{"] List [{";" List}] ["}"] [")"]}
+Expression    =     ([{"("}]  SubExpr [{Operator [{"("}] SubExpr [{")"}]}] [{")"}] | {["("] ["{"] List [{";" List}] ["}"] [")"]}) 
 SubExpr       =     Token [{Operator Token}]
 Token         =     [{Unary}] Argument [(Operator | Function) ["("] [{Unary}] [Argument] [")"]]
 Argument      =     (List | Variable | Operand)