Fix release_nested_message typo in C++ arenas guide

[Hashim1999164]

Summary

In the inefficient arena release_...() example, the sample called release_message() even though the field used throughout the snippet is nested_message.

This updates that call to release_nested_message() so it matches set_allocated_nested_message and the unsafe arena APIs in the same section.

Fixes protocolbuffers/protobuf#28830

Test plan

• Confirmed the surrounding examples already use release_nested_message / unsafe_arena_release_nested_message
• Docs preview renders the corrected code block

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[shaod2]

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@Hashim1999164 please fix

[Hashim1999164]

@googlebot I signed it!

[Hashim1999164]

recheck

[Hashim1999164]

@googlebot I signed it!