Correct high-symmetry points example graphene

[PabloPiaggi]

I am correcting the high-symmetry points in the graphene example. According to seekpath the high symmetry points for this primitive cell are:

Label	k1	k2	k3
Γ	0.0000000000	0.0000000000	0.0000000000
M	0.5000000000	0.0000000000	0.0000000000
K	0.3333333333	0.3333333333	0.0000000000

Old version results:

New version results:

The new version results match the data generated by the graphene example here.

[abelcarreras]

Oh, thanks for the correction!