Changed LaTex $\bot$s to ⊥
Changed LaTex $\bot$s to ⊥

In the HTML version of the documentation, it isn't rendered so might as well use the unicode representation.
Part of the problem was that putting a math unicode character wasn't
rendering properly in the pdf, so extra steps were needed to define
the unicode charecter ⊥ in reference.tex

closes #15285

rust-lang/reference,rust-lang/reference