docs: fix minor wording issue in security section (#422)

Author: annwag

SECURITY.md

@@ -22,7 +22,7 @@ platform.
 The HackerOne platform allows us to have a centralized and single reporting
 source for us to deliver optimized SLA's and results. All reports submitted to
 the platform are triaged around the clock by our team of Coinbase engineers
-with domain knowledge, assuring the best quality of review.
+with domain knowledge, ensuring the best quality of review.
 
 For more information on reporting vulnerabilities and our HackerOne bug bounty
 program, view our [security program policies][7].