Remove an extra "|" from the definition of \xasm

[mik-jozef]

Renders eg. in 3.2 Assemblies as line 0, I suppose the intended rendering is as in line 1.

S = (|S||, ||S||, 푆_S) where |S| is its underlying set ...
S = (|S|, ||S||, 푆_S) where |S| is its underlying set ...