How to use a 100 amp sub panel for a 50 amp input
Just wire it up.  There is no problem using a sub panel that can take more current than you will ever feed it.

Points to note:


The cable to your hot tub will be rated for 50A (not 100A); therefore you must not upgrade the breaker in your main panel to 100A.
It would be sensible to add a note near the main panel that the cable is only rated for 50A, so not to change the breaker (so you, or a future purchaser, remembers in a year's time).
If you connect four 20A 240V devices to the sub-panel in your shed (and run them all at once) the sub-panel will be entirely happy with this, but the 50A breaker in the main panel won't be and it will throw - so don't do that then.


PS.  This is not directly related to your question, but still. 50A is a substantial current.  Do remember to torque the connection to the panel correctly.