Fix 'sub class' -> 'subclass' spelling in README.md

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: dblock

README.md

@@ -232,7 +232,7 @@ The `DuplicateValueError` exception is raised to be consistent with the unique k
 
 ### Inheritance
 
-When inheriting from a `Ruby::Enum` class, all defined enums in the parent class will be accessible in sub classes as well. Sub classes can also provide extra enums, as usual.
+When inheriting from a `Ruby::Enum` class, all defined enums in the parent class will be accessible in subclasses as well. Subclasses can also provide extra enums, as usual.
 
 ``` ruby
 class OrderState
@@ -270,7 +270,7 @@ ShippedOrderState.key?(:CREATED) # true
 ShippedOrderState.value(:CREATED) # 'CREATED'
 ```
 
-A sub class may redefine a key or value already used by a parent class without raising `DuplicateKeyError` or `DuplicateValueError`; its own definition takes precedence.
+A subclass may redefine a key or value already used by a parent class without raising `DuplicateKeyError` or `DuplicateValueError`; its own definition takes precedence.
 
 ``` ruby
 class ShippedOrderState < OrderState