Fix a string that should be an f-string.

Co-authored-by: Beth <167304066+bprobert97@users.noreply.github.com>

Author: rwb27

src/labthings_fastapi/thing_slots.py

@@ -215,7 +215,7 @@ def _pick_things(
                 if not isinstance(things[t], self.thing_type):
                     raise ThingSlotError(f"{t} is the wrong type")
             return [things[t] for t in target]
-        msg = "The target specified for a ThingSlot ({target}) has the wrong "
+        msg = f"The target specified for a ThingSlot ({target}) has the wrong "
         msg += "type. See ThingSlot.connect() docstring for details."
         raise TypeError(msg)