[SPARK-8395] [DOCS] start-slave.sh docs incorrect

start-slave.sh no longer takes a worker # param in 1.4+

Author: Sean Owen <sowen@cloudera.com>

Closes #6855 from srowen/SPARK-8395 and squashes the following commits:

300278e [Sean Owen] start-slave.sh no longer takes a worker # param in 1.4+

docs/spark-standalone.md

@@ -24,7 +24,7 @@ the master's web UI, which is [http://localhost:8080](http://localhost:8080) by
 
 Similarly, you can start one or more workers and connect them to the master via:
 
-    ./sbin/start-slave.sh <worker#> <master-spark-URL>
+    ./sbin/start-slave.sh <master-spark-URL>
 
 Once you have started a worker, look at the master's web UI ([http://localhost:8080](http://localhost:8080) by default).
 You should see the new node listed there, along with its number of CPUs and memory (minus one gigabyte left for the OS).