Update SensirionShdlcTxFrame.cpp

the begin function sets the index to 0.

src/SensirionShdlcTxFrame.cpp

@@ -38,7 +38,7 @@
 
 uint16_t SensirionShdlcTxFrame::begin(uint8_t command, uint8_t address,
                                       uint8_t dataLength) {
-    _index=0;                          
+    _index=0;                   //sets the index to point towards the start of the buffer.       
     _buffer[_index] = 0x7e;
     uint16_t error = addUInt8(address);
     error |= addUInt8(command);