Update allwplib_subtree.sh's directions to actually work

Change-Id: Iedc8c9b3f16700b945a1ae7a8e75276f372b166f
diff --git a/doc/allwpilib_subtree.sh b/doc/allwpilib_subtree.sh
index 87b95d9..aa83d0c 100755
--- a/doc/allwpilib_subtree.sh
+++ b/doc/allwpilib_subtree.sh
@@ -15,7 +15,7 @@
 set -o pipefail
 
 if [ $# -ne 4 ]; then
-  echo "Usage: $0 add|pull|filter prefix remote ref" >&2
+  echo "Usage: $0 add|merge|filter prefix remote ref" >&2
   exit 1
 fi