docs: correct default value for DD_TRACE_128_BIT_TRACEID_GENERATION_ENABLED config

[frrahat]

addresses #11905

As per documentation default value for DD_TRACE_128_BIT_TRACEID_GENERATION_ENABLED is False:

dd-trace-py/docs/configuration.rst

Lines 121 to 123 in d0b151a

	DD_TRACE_128_BIT_TRACEID_GENERATION_ENABLED:
	type: Boolean
	default: False

But in code it is being passed as True:

dd-trace-py/ddtrace/settings/config.py

Line 515 in d0b151a

self._128_bit_trace_id_enabled = _get_config("DD_TRACE_128_BIT_TRACEID_GENERATION_ENABLED", True, asbool)

I think it was introduced by this: #7181 and True as default is the expected here. So updated the documentation accordingly in this PR.

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[frrahat]

I think we need to add "changelog/no-changelog" label, right? cc: @sanchda