Update binaryformatter-security-guide.md

[GrabYourPitchforks]

Summary

Reworded a portion of the BinaryFormatter security guide after consultation with the security LT.

• Removed the word "vulnerability" from a key paragraph. We document that BF is only intended to be used with trusted input, so we would not consider any undesired behavior caused by passing untrusted input to it to be a vulnerability.
• Clarified that the .NET team is not committed to making code changes in response to binder bypasses or other exploits. The previous wording was somewhat ambiguous and could have been incorrectly interpreted as that .NET will still try to make code changes except when impractical to do so.

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docs/standard/serialization/binaryformatter-security-guide.md	Deserialization risks in use of BinaryFormatter and related types

[gewarren]

I left some style suggestions.

[GrabYourPitchforks]

Thanks for the feedback! :)