Backport PR #29995: Fix typo: missing singlequote in unrecognized backend exception

Author: dstansby

lib/matplotlib/backends/registry.py

@@ -407,7 +407,7 @@ def resolve_gui_or_backend(self, gui_or_backend):
             return self.resolve_backend(gui_or_backend)
         except Exception:  # KeyError ?
             raise RuntimeError(
-                f"'{gui_or_backend} is not a recognised GUI loop or backend name")
+                f"'{gui_or_backend}' is not a recognised GUI loop or backend name")
 
 
 # Singleton