Apply suggestion from @tashian

Author: tashian

step-ca/acme-basics.mdx

@@ -49,8 +49,8 @@ However, ACME can't determine whether a client
 is the rightful owner of the identifier,
 or merely an entity currently controlling it.
 
-Therefore, ACME's security model leans heavily on DNS and network security,
-an ACME administrator must ensure
+ACME's security model relies heavily on DNS and network security.
+An ACME administrator must ensure
 that host IP assignment and DNS resolution
 are appropriately secured.
 And in some organizations,