[css-color-4] Remove non-2119 use of 'should' in privsec section. Fixes w3c#4439.

Author: tabatkins

css-color-4/Overview.bs

@@ -3022,4 +3022,4 @@ also pose a security risk,
 as they make it easier for a malware site
 to create user interfaces that appear to be from the system.
 However, as several system colors are now defined to be "generic",
-this risk should be mitigated.
+this risk is believed to be mitigated.