documentation missing factor in TSL eq 81

[andrewmholcomb]

Eq. 81 in section 5.11.2 is missing a factor of 2. Code is correct, only a documentation bug.

openmc/src/secondary_thermal.cpp

Line 50 in 2efe223

mu = 1.0 - 2.0 * energies[k] / E_in;

in docs/source/methods/neutron_physics.rst
this: "\mu = 1 - \frac{E_i}{E}"
should be "\mu = 1 - \frac{2E_i}{E}"

[paulromano]

Thanks for pointing that out @andrewmholcomb! I just added a fix as part of pull request #2081.