The tip is still true since no champion has an innate 45% cooldown reduction but it should be reworded.
''You can only get a maximum of 40% cooldown reduction, but the intelligence mastery raises the cap to 45%''
Edit:
Since some don't get it
40% is the maximum amount any champion can have at all times without any external sources.
45% is the maximum amount any champion can have under certain circumstances using an external source.
40% and 45% are both the correct answers, but 45% is circumstantial so it is not completely true, but since 40% can be raised, is not completely true either.
Both 40% and 45% contradict one another so both are true and false if used alone, hence why you would change the tip to include both numbers.