fix(doc): brace sub-script for tex4ht

Author: jodavies

doc/manual/variable.tex

@@ -186,7 +186,7 @@ \section{Symbols}
 the natural logarithm $e$.
 
 \noindent em\_\index{em\_}: a reserved variable which indicates the 
-Euler-Mascheroni constant $\gamma_\mathrm{E}$.
+Euler-Mascheroni constant $\gamma_{\mathrm{E}}$.
 
 \noindent coeff\_\index{coeff\_}: this variable is automatically replaced 
 by the coefficient of the current term.