fix: add missing import in example code (#805)

Co-authored-by: Marcelo Trylesinski <[email protected]>

Author: ltdt-apex

docs/concepts.md

@@ -43,6 +43,10 @@ In this example:
 3. Logfire calculates the age from the `dob` and displays age in the debug message
 
 ```py
+from datetime import date
+
+import logfire
+
 logfire.configure()
 
 with logfire.span('Asking the user for their {question}', question='birthday'):  # (1)!