docs: fix minor wording issue in security section

[annwag]

I updated the phrase "assuring the best quality of review" to "ensuring the best quality of review" for better clarity and accuracy. "Ensure" is more appropriate here, as it emphasizes guaranteeing the outcome of the review process.

[cb-heimdall]

✅ Heimdall Review Status

Requirement	Status	More Info
Reviews	✅ 1/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1