Fix watermark stat display title as "Buffer Pool" instead of "Shared Pool"

[rck-innovium]

"show buffer_pool watermark" reads SAI_BUFFER_POOL_STAT_WATERMARK_BYTES. This corresponds to the total buffer occupancy and not the shared buffer pool occupancy.

fixes #1321

- What I did
Fixed the display string in the watermarkstat script.

- How to verify it
show buffer_pool watermark

- Previous command output (if the output of a command-line utility has changed)

$ show buffer_pool watermark
Shared pool maximum occupancy:
                 Pool    Bytes
---------------------  -------
ingress_lossless_pool        0
           lossy_pool     4032

- New command output (if the output of a command-line utility has changed)

$ show buffer_pool watermark
Buffer pool maximum occupancy:
                 Pool    Bytes
---------------------  -------
ingress_lossless_pool        0
           lossy_pool     1568

[ghost]

All CLA requirements met.

[rck-innovium]

retest please

[rck-innovium]

Retest this please.

[rck-innovium]

Retest this please.

[lguohan]

can you resolve the conflict?