fix(09_Constant): initialize IMMUTABLE_BLOCK

[auyua9]

What changed

The Chinese 09_Constant example assigns IMMUTABLE_NUM again in the constructor while leaving IMMUTABLE_BLOCK uninitialized. This causes the example to fail compilation under Solidity 0.8.x and disagrees with the working localized examples.

This PR replaces the erroneous constructor assignment with IMMUTABLE_BLOCK = block.number and updates the matching README code block.

Scope

• 09_Constant/Constant.sol
• 09_Constant/readme.md

No runtime dependency or unrelated cleanup is included.

Verification

• git diff --check
• Compared the corrected initialization with Languages/en, Languages/es, Languages/ja, and Languages/pt-br versions
• Local forge/solc binaries were unavailable for a compile run