awk: add print every nth line example (#2008)

pages/common/awk.md

@@ -21,3 +21,7 @@
 - Sum the values in the first column and pretty-print the values and then the total:
 
 `awk '{s+=$1; print $1} END {print "--------"; print s}' {{filename}}`
+
+- Print every third line starting from the first line:
+
+`awk 'NR%3==1' {{filename}}`