Add cross-reference to 'exposed' in the [SecureContext] definition

Fixes #119.

Author: Ms2ger

index.bs

@@ -9870,7 +9870,7 @@ If the [{{SecureContext}}] [=extended attribute=] appears on an
 [=interface member=],
 [=interface mixin member=], or
 [=namespace member=],
-it indicates that the construct is exposed
+it indicates that the construct is [=exposed=]
 only within a [=secure context=].
 The [{{SecureContext}}] extended attribute must not be used
 on any other construct.