fixed filename in build_docs.sh

Author: pcampr

scripts/build_docs.sh

@@ -3,7 +3,7 @@
 
 PORT=${1:-4000}
 
-echo "usage: build.sh [port]"
+echo "usage: build_docs.sh [port]"
 
 # Find the docs dir, no matter where the script is called
 ROOT_DIR="$( cd "$(dirname "$0")"/.. ; pwd -P )"