use american spelling of neighbor consistently in docs and code comments

doc/src/fix_langevin_drude.txt

@@ -154,7 +154,7 @@ Note: The temperature thermostating the core-Drude particle pairs
 should be chosen low enough, so as to mimic as closely as possible the
 self-consistent minimization. It must however be high enough, so that
 the dipoles can follow the local electric field exerted by the
-neighbouring atoms. The optimal value probably depends on the
+neighboring atoms. The optimal value probably depends on the
 temperature of the centers of mass and on the mass of the Drude
 particles.
 

doc/src/pair_kolmogorov_crespi_z.txt

@@ -38,7 +38,7 @@ This shift is achieved by the last term in the equation for {Vij} above.
 This potential is intended for interactions between two layers of graphene.
 Therefore, to avoid interaction between layers in multi-layered materials,
 each layer should have a separate atom type and interactions should only
-be computed between atom types of neighbouring layers.
+be computed between atom types of neighboring layers.
 
 The parameter file (e.g. CC.KC), is intended for use with metal
 "units"_units.html, with energies in meV. An additional parameter, {S},