SAMZA-2640: Fixing errors in WindowType examples in High Level API doc

[ajothomas]

Issue:
The Window Types section in the High Level API documentation has a few mistakes:

1. Missing variable name for a Tumbling window example
2. Incorrect example for Session windows (seems like a copy paste issue from Tumbling window example)

Changes:

1. Added missing variable name for the Tumbling window example
2. Incorrect code was removed from the Session window section and a new example was added in its place.

Tests
NA. Since this is only a doc fix.

[Sanil15]

Thanks for the fix! Please follow guidelines here for naming, description of PR: https://cwiki.apache.org/confluence/display/SAMZA/SEP-25%3A+PR+Title+And+Description+Guidelines