Fix pdf build of pair_atm docs

doc/src/pair_atm.rst

@@ -61,7 +61,7 @@ command as in the example above.
 The potential for a triplet of atom is calculated only if all 3 distances
 :math:`r_{12}`, :math:`r_{23}`, :math:`r_{31}` between the 3 atoms satisfy
 :math:`r_{IJ} < \text{cutoff}`.  In addition, the product of the 3 distances
-:math:`r_{12} r_{23} r_{31} < \text{cutoff_triple}^3` is required, which
+:math:`r_{12} r_{23} r_{31}` < cutoff_triple :math:`^3` is required, which
 excludes from calculation the triplets with small contribution to the
 interaction.