Update electrostatic.rst

doc/src/input/systems/interactions/electrostatic.rst

@@ -60,7 +60,7 @@ distribution (like a crystal) a lower value, such as 5 is sufficient, and for
 more heterogeneous system, higher values of ``kmax`` are needed.
 
 It is also possible (but not required) to set the ``alpha`` parameter of Ewald
-solver directly with the corresponding keyword. A good value of ``alpha`` one
+solver directly with the corresponding keyword. A good value of ``alpha`` is one
 that satisfies :math:`\exp \left(-\alpha \frac L 2 \right) << 1`. If no value is
 provided in the input file, the default value of :math:`\pi / \text{cutoff}` is
 used.