Make clearer how one updates the graph after changing a gain value about pidtuner.github.io

[pidtuner]

Hi George,

Thank you for the great feedback (pun intended), I did not realize this obvious UI flaw. But you are right, when typing enter might be the first intuitive thing to do for one person, it might not be the other. I will consider finding a more user-friendy way to update the graph for the next release (when I find the time).

Let us know how your tuning goes. Happy tuning!