Fix typo

VOHE-Note.tex

@@ -129,7 +129,7 @@ \subsubsection{H.E.S.S}
 the astrophysical source properties. To derive them from the event lists, models of background due to the remaining CRs
 (generally generated from real observations) are used with the gamma-ray IRFs (PSF, Energy Dispersion, Collection Area)
 that are generated by extensive Monte Carlo simulations. These 4 IRFs (background, PSF, Edisp, CollArea) are computed
-for each observation of \sim~30min and are valid for the field of view. They depend on true energies, positions in the
+for each observation of $\sim$~30min and are valid for the field of view. They depend on true energies, positions in the
 field of view and sometimes from event classification types. The derivation of astrophysical quantities from
 the event lists are now using open libraries, in particular the reference library Gammapy \ref{gammapy:2023}.
 %% Need to describe the IRFs like for Chandra?