fix SECURITY.md

[NishantRana07]

Proposed Changes:

Updated security.md for better readability and security

[CLAassistant]

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[vblagoje]

@silvanocerza can you take over this one please?

[silvanocerza]

The Haystack team fully owns the security policy and we see no need to update it as of now. Thank you for your contribution in any case.