Update awk_sed.md

awk_sed.md

@@ -1,9 +1,15 @@
-### AWK and SED Tips and tricks
+# AWK and SED Tips and tricks
 
 ## AWK
 
-# Calculate total or average
+### Calculate total or average
 
 ```sh
 awk '{sum+=$1; n++} END {print sum / n}'
-```
+```
+
+or using builtin row counter NR
+
+```sh
+awk '{sum+=$1} END {{if NR > 0} print sum / NR}'
+```