Fix the missing return type (Azure#28581)

Author: xiangyan99

sdk/core/azure-core/azure/core/tracing/_abstract_span.py

@@ -169,7 +169,7 @@ def set_current_span(cls, span: Any) -> None:
         """
 
     @classmethod
-    def set_current_tracer(cls, tracer: Any):
+    def set_current_tracer(cls, tracer: Any) -> None:
         """
         Set the given tracer as the current tracer in the execution context.
         """