Measure Vcc on ATtiny13a?
The code you linked to measures the 1.1 V internal reference using
Vcc as a reference. The ATtiny13A cannot do that. It does have a 1.1 V
internal reference, but it can only use it as a reference for the ADC,
not as a measured channel.
What you can do, if you don't mind adding extra hardware, is divide Vcc
with some resistors in order to get a voltage proportional to Vcc but
lower than 1.1 V. Then use the 1.1 V reference to measure that
voltage. In other words, measure a scaled-down Vcc against the
1.1 V reference instead of measuring the 1.1 V reference
against Vcc. The datasheet should help you with the details.