Remove extra --build-arg argument for Docker build command (#24)

diff --git a/README.md b/README.md
index 4b5af52..6c709fe 100644
--- a/README.md
+++ b/README.md
@@ -39,7 +39,7 @@
 
    Or build with an Accumulo tarball (located in same directory as DockerFile) using the command below:
 
-        docker build --build-arg --build-arg ACCUMULO_FILE=accumulo-2.0.0-SNAPSHOT-bin.tar.gz -t accumulo .
+        docker build --build-arg ACCUMULO_FILE=accumulo-2.1.0-SNAPSHOT-bin.tar.gz -t accumulo .
 
 ## Image basics