If it's a sure thing, it doesn't matter what the odds are - it's found money. In a larger way, I'd put my money on Mueller over the bookies.
5/4 are long odds. It means the bookies are NOT betting on it. If they were betting on it, it would “odds on” eg. 1/3. If they didn’t have a clue it would be evens. They know it isn’t happening so are offering good prices if it does. These odds are tempting if you are determined he will get impeached and have convinced yourself despite the 2/3 senate majority required to do it.