Fix LaTeX functions with underscore (#1330)

Author: hollasch

books/RayTracingTheRestOfYourLife.html

@@ -1083,7 +1083,7 @@
 For our purposes, if we have PDF $p()$ and cumulative distribution function $P()$, we can use this
 "inverse function" with a random number to get what we want:
 
-    $$ f(d) = P^{-1} (\operatorname{random_double}()) $$
+    $$ f(d) = P^{-1} (\operatorname{random\_double}()) $$
 
 For our PDF $p(r) = r/2$, and corresponding $P(r)$, we need to compute the inverse of $P(r)$. If we
 have
@@ -1100,7 +1100,7 @@
 
 Thus our random number generator with density $p(r)$ can be created with:
 
-    $$ f(d) = \sqrt{4\cdot\operatorname{random_double}()} $$
+    $$ f(d) = \sqrt{4 \cdot \operatorname{random\_double}()} $$
 
 Note that this ranges from 0 to 2 as we hoped, and if we check our work, we replace
 `random_double()` with $1/4$ to get 1, and also replace with $1/2$ to get $\sqrt{2}$, just as
@@ -2154,7 +2154,7 @@
 cross product that $\mathbf{n} \times \mathbf{a}$ is perpendicular to both $\mathbf{n}$ and
 $\mathbf{a}$:
 
-    $$ \mathbf{s} = \operatorname{unit_vector}(\mathbf{n} \times \mathbf{a}) $$
+    $$ \mathbf{s} = \operatorname{unit\_vector}(\mathbf{n} \times \mathbf{a}) $$
 
     $$ \mathbf{t} = \mathbf{n} \times \mathbf{s} $$