fix format bug for elsestme and stmt def in .g4

src/SysY.g4

@@ -137,6 +137,7 @@ blockStmt: LBRACE blockItem* RBRACE;
 blockItem: decl | stmt;
 
 stmt: lValue ASSIGN exp SEMICOLON
+		| exp? SEMICOLON
 		| blockStmt
 		| IF LPAREN cond RPAREN stmt (ELSE stmt)?
 		| WHILE LPAREN cond RPAREN stmt