Fix syntax for the visibility function term (KhronosGroup#2400)

Author: lexaknyazev

specification/2.0/Specification.adoc

@@ -3058,7 +3058,7 @@ G = \frac{2 \, \left| N \cdot L \right| \, \chi^{+}(H \cdot L)}{\left| N \cdot L
 
 where χ^+^(*x*) denotes the Heaviside function: 1 if *x* > 0 and 0 if *x* <= 0. See <<Heitz2014,Heitz (2014)>> for a derivation of the formulas.
 
-Introducing the visibility function $\mathcal{V}$
+Introducing the visibility function latexmath:[\mathcal{V}]
 
 [latexmath]
 ++++