blockifier_reexecution: simplify README wording per Orwell style rules

[dan-starkware]

Prose-only edits to crates/blockifier_reexecution/README.md, applying Orwell's writing rules. No facts, numbers, or names changed.

• Line 38: It is necessary to supply a RPC provider url → You must supply a RPC provider url.
• Line 75: passive → active should be run → Run offline reexecution …; cut very from very long.

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https://claude.ai/code/session_01GjrAzEXJmrgsDjg9YSA1c6

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PR Summary

Low Risk
Markdown-only edits to documentation; no runtime, security, or API impact.

Overview
Documentation-only updates to crates/blockifier_reexecution/README.md under RPC test and Offline Reexecution—no behavior, CLI, or code changes.

In the RPC test section, the requirement to provide an RPC URL is stated directly as “You must supply a RPC provider url” instead of the more formal “It is necessary to supply…” phrasing.

For offline reexecution, the instructions use an imperative (“Run offline reexecution in release mode”) rather than passive “should be run”, and “very long” is shortened to “long.”

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[reviewable-StarkWare]

This change is