correct typo in write_dump docs

this closes #233

doc/src/write_dump.txt

@@ -26,7 +26,7 @@ write_dump all atom dump.atom
 write_dump subgroup atom dump.run.bin
 write_dump all custom dump.myforce.* id type x y vx fx
 write_dump flow custom dump.%.myforce id type c_myF\[3\] v_ke modify sort id
-write_dump all xyz system.xyz modify sort id elements O H
+write_dump all xyz system.xyz modify sort id element O H
 write_dump all image snap*.jpg type type size 960 960 modify backcolor white
 write_dump all image snap*.jpg element element &
    bond atom 0.3 shiny 0.1 ssao yes 6345 0.2 size 1600 1600  &