README: Clarified SetCustomMeta

Author: sebnilsson

README.md

@@ -40,7 +40,7 @@ where `{0}` is the value from `PageTitle` and `{1}` is the value from `SiteName`
 ### Methods
 
 - `SetCustomMeta(string key, string value, CustomMetaAttributeKey? attribute = null)`: Add any
-  custom meta-tag. Keys starting with prefixes such as `og:` or `fb:` default to the `property` attribute. Use
+  custom meta-tag. Keys starting with known prefixes such as `og:` or `fb:` default to the `property` attribute. Use
   the `attribute` parameter to explicitly choose `name` or `property`.
 - `SetMetaRobots(bool index, bool follow)`: Specify the instructions for robots.
 Updates the value for `MetaRobots`.