Merge bitcoindevkit/bdk#643: Fix README.md link to rust 1.56.1 blog post

d020dede3785f351c0a58a0787c20c396437ff87 Fix README.md link to rust 1.56.1 blog post (Steve Myers)

Pull request description:

  ### Description

  Fix link to rust blog for 1.56.1 as pointed out by ulrichard: https://github.com/weareseba/bdk-reserves/pull/5#issuecomment-1159212838

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