README : Incorrect description about behavior of scx_XXX scheduler

[yushoyamaguchi]

Currently, there is a statement in the README that if you install scx_XXX, it means that all tasks will be moved there.

scx/README.md

Line 67 in 274bcf7

Above, we switch the whole system to use `scx_simple` by running the binary,

However, in this commit, I thought the change was made so that simply installing scx_XXX would not move the task to this scheduler?

If so, I felt the README description should be changed.
I apologize if I have missed something.

ref : https://lore.kernel.org/r/all/[email protected]/T/