fix section heading ref

Author: RexJaeschke

standard/classes.md

@@ -3452,7 +3452,7 @@ In a *ref_property_body* an expression body consisting of `=>` followed by `ref`
 
 When a property declaration includes an `extern` modifier, the property is said to be an ***external property***. Because an external property declaration provides no actual implementation, each of the *accessor_body*s in its *accessor_declarations* shall be a semicolon.
 
-The modifier `required` indicates the instance member being declared is required to be set during object initialization, which forces the instance creator to provide an initial value for the member in an object initializer at the creation site. (See  [§12.8.20](expressions.md#12820-default-value-expressions) and §SetsRequiredMembersAttribute for exemptions to this requirement.) A required member shall not be static. A required member shall be at least as accessible as its containing type.
+The modifier `required` indicates the instance member being declared is required to be set during object initialization, which forces the instance creator to provide an initial value for the member in an object initializer at the creation site. (See  [§12.8.20](expressions.md#12820-default-value-expressions) and §SetsRequiredMembers for exemptions to this requirement.) A required member shall not be static. A required member shall be at least as accessible as its containing type.
 
 > *Note*: Although a required member declaration may include an initializer (*property_initializer* for a property, *variable_initializer* for a field), ordinarily, that initializer serves no purpose, as the instance creator is required to provide an initial value for that member anyway. However, if a constructor is decorated with the `SetsRequiredMembers` attribute the compiler assumes that member has been initialized correctly, and will not require an explicit initializer by the instance creator, resulting in the member’s initial value being that of its initializer, if one is present. *end note*