[PROOFREADING] pos305 - ja #4439
Description
en PBN version: https://planb.academy/ja/courses/pos305-6fc12131-e464-4515-9d3f-9255365d5fa1
en github version: https://github.com/PlanB-Network/bitcoin-educational-content/blob/dev/courses/pos305/en.md
ja github version: https://github.com/PlanB-Network/bitcoin-educational-content/blob/dev/courses/pos305/ja.md