rb_decay documentation typos in equations.

[NaucklearPhysicist]

The current docstring for rb_decay misses some brackets for proper exponentials.

The equation

y'(x+dx) = a \alpha^x \alpha^dx.
should be y'(x+dx) = a \alpha^x \alpha^{dx}, so both d and x are in the exponent.

The same thing applies in the next equation, where

ry = \frac{a \alpha^x \alpha^dx}{a \alpha^x} = \alpha^dx.
should be ry = \frac{a \alpha^x \alpha^{dx}}{a \alpha^x} = \alpha^{dx}.

Additionally, I would omit the

= y(x) - b

in the second equation. The statement is correct, but I don't see the benefit of stating it for this documentation.