fix compilation warning in Text::[]

src/lib/include/jes/Text.h

@@ -15,7 +15,7 @@ namespace jes
         Text(const uint8_t buf[], size_t size);
         size_t get_size() { return m_size - m_gap_size; }
 
-        Character operator[](int index)
+        Character operator[](size_t index)
         {
             return (Character)(m_buffer[index < m_gap_index ? index : index + m_gap_size]);
         }