fix typo

interpreter/prim.c

@@ -244,7 +244,7 @@ void __string_slice(int dst_idx)
         chars = str->_array._chars;
     }
 
-    int length = idx2->_int - idx1->_int + 1;
+    int length = idx2->_int - idx1->_int;
     
     target = string_create(target, length);
     memcpy(target->_array._chars, chars + idx1->_int, length);