[rand.eng.philox] Use normal font for superscript of "ith"

Author: jwakely

source/numerics.tex

@@ -3177,7 +3177,7 @@
 
 \pnum
 The generation algorithm returns $Y_i$,
-the value stored in the $i^{th}$ element of $Y$ after applying
+the value stored in the $i^\text{th}$ element of $Y$ after applying
 the transition algorithm.
 
 \pnum