Adjust prompts in create reward form

[M3L6H]

The prompt for the reward in the create reward form is currently: "What to do when you're done?" It should be changed simply to "Reward". Then the placeholder should contain some actual examples of rewards, i.e. "Play video games, watch a movie, etc."

The prompt for petals in the create reward form is currently: "How many petals will you need for this reward?" It should be changed to something along the lines of "Petal cost for redeeming this reward".