Update column name for pgtle functions

[Rajanpandey]

Issue #, if available: None

Description of changes: Updated 03_managing_extensions.md to reflect the correct column names for pgtle.available_extensions() and pgtle.available_extension_versions().

As shown below, the column name is comment:

test=# SELECT * FROM pgtle.available_extensions();
    name     | default_version |              comment              
-------------+-----------------+-----------------------------------```

and

test=# SELECT * FROM pgtle.available_extension_versions();
    name     | version | superuser | trusted | relocatable | schema | requires |              comment              
-------------+---------+-----------+---------+-------------+--------+----------+-----------------------------------

But in the doc, it said description.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[adamguo0]

Thanks for catching this!