Fix typo in Butler-Volmer equation docstring (#5279)

Author: cnaples79

src/pybamm/models/submodels/interface/kinetics/butler_volmer.py

@@ -12,7 +12,7 @@ class SymmetricButlerVolmer(BaseKinetics):
     Submodel which implements the symmetric forward Butler-Volmer equation:
 
     .. math::
-        j = 2 * j_0(c) * \\sinh(ne * F * \\eta_r(c) / RT)
+        j = 2 * j_0(c) * \\sinh(ne * F * \\eta_r(c) / 2RT)
 
     Parameters
     ----------