Wrong configuration key for no_shard_specified_behavior in default pgcat.toml

[nothrow]

Describe the bug
PR #578 added configurable behavior for situation, when no shard is selected.

It is added into example pgcat.toml, but under wrong key. It kind of confused me.

The # no_shard_specified_behavior = shoud be # default_shard =

Expected behavior
No problem in behavior. Just improvement in example configuration.