You could get struck by lightning in the shower but the chances of getting struck by lightning taking a shower are very low.
In the old days steel and other metal pipes were used to plumb a house and if lightning struck those pipes then the lightning could travel into your house and strike you when taking a shower or using the water.
It still possible though today especially if your home has copper pipes and even with plastic pipes the lightning could if it wanted to travel through the water itself and strike you in the shower.
Although the chances of getting struck by lightning in the shower are very low I still wouldn't risk it as a shower can wait until after the lightning storm is over and it's not worth taking the risk and potentially getting injured or killed over a shower.
I always just wait until after the lightning storm to take a shower not only because of the lightning but also because of a power failure that may happen and leave my bathroom in the dark.