Fix SDF documentation error

Author: tcmoore3

hoomd/hpmc/compute.py

@@ -441,8 +441,8 @@ def betaP(self):  # noqa: N802 - allow function name
         required) and computes the pressure via:
 
         .. math::
-            \\beta P = \\rho \\left(1 + \\frac{s_\\mathrm{comp}(0+)}{2d} +
-            \\frac{s_\\mathrm{exp}(0-)}{2d} \
+            \\beta P = \\rho \\left(1 +
+            \\frac{s_\\mathrm{comp}(0+) - s_\\mathrm{exp}(0-)}{2d} \
             \\right)
 
         where :math:`d` is the dimensionality of the system, :math:`\\rho` is