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 66107f46f374f83729cd79ab260eb59fa123c041)
Signed-off-by: Andrew Or <andrewor14@gmail.com>