Docs : use "--total-executor-cores" rather than "--cores" after spark-shell

Author: CrazyJvm <crazyjvm@gmail.com>

Closes #2540 from CrazyJvm/standalone-core and squashes the following commits:

66d9fc6 [CrazyJvm] use "--total-executor-cores" rather than "--cores" after spark-shell

(cherry picked from commit 66107f46)
Signed-off-by: Andrew Or <andrewor14@gmail.com>

docs/spark-standalone.md

@@ -242,7 +242,7 @@ To run an interactive Spark shell against the cluster, run the following command
 
     ./bin/spark-shell --master spark://IP:PORT
 
-You can also pass an option `--cores <numCores>` to control the number of cores that spark-shell uses on the cluster.
+You can also pass an option `--total-executor-cores <numCores>` to control the number of cores that spark-shell uses on the cluster.
 
 # Launching Compiled Spark Applications