Correct type hint: namespace is not optional

The namespace argument is positional and will never be `None` so should
not be marked as Optional. It is always a string, and the default is
to pass in an empty string.

README.md

@@ -121,7 +121,7 @@ take effect globally.
 ```python
 def my_key_builder(
         func,
-        namespace: Optional[str] = "",
+        namespace: str = "",
         request: Request = None,
         response: Response = None,
         *args,

fastapi_cache/key_builder.py

@@ -7,7 +7,7 @@ from starlette.responses import Response
 
 def default_key_builder(
     func: Callable,
-    namespace: Optional[str] = "",
+    namespace: str = "",
     request: Optional[Request] = None,
     response: Optional[Response] = None,
     args: Optional[tuple] = None,