Added '@' to handles

DATA_MAINTAINERS.md

@@ -1,9 +1,9 @@
 # Data maintainers
 
 Contacts (GitHub handles):
-- kingst
-- marv-out
-- mikeplante1
-- dsnallfot
-- bjornoleh
-- dnzxy
+- @kingst
+- @marv-out
+- @mikeplante1
+- @dsnallfot
+- @bjornoleh
+- @dnzxy