Fixed typo in "compute_voronoi_atom.txt"

There was the typo "requiers" in the Restrictions section of compute_voronoi_atom.txt.

doc/src/compute_voronoi_atom.txt

@@ -217,7 +217,7 @@ This compute is part of the VORONOI package.  It is only enabled if
 LAMMPS was built with that package.  See the "Making
 LAMMPS"_Section_start.html#start_3 section for more info.
 
-It also requiers you have a copy of the Voro++ library built and
+It also requires you have a copy of the Voro++ library built and
 installed on your system.  See instructions on obtaining and
 installing the Voro++ software in the src/VORONOI/README file.