Small correction in Streaming Programming Guide doc

Corrected description of `repartition` function under 'Level of Parallelism in Data Receiving'.

Author: akkomar <ak.komar@gmail.com>

Closes #1079 from akkomar/streaming-guide-doc and squashes the following commits:

32dfc62 [akkomar] Corrected description of `repartition` function under 'Level of Parallelism in Data Receiving'.

(cherry picked from commit edb1f0e3164b99b483b6878efe0b6ea5ae9f97ed)
Signed-off-by: Reynold Xin <rxin@apache.org>