chore: tweak error message about weak options

Author: kim-em

src/Lean/Language/Lean.lean

@@ -315,7 +315,7 @@ def reparseOptions (opts : Options) : IO Options := do
       | unless weak do
           throw <| .userError s!"invalid -D parameter, unknown configuration option '{name}'
 
-If the option is defined in this library, use '-D{`weak ++ name}' to set it conditionally"
+If the option is defined in a library, use '-D{`weak ++ name}' to set it conditionally"
 
     let .ofString val := val
       | opts' := opts'.insert name val  -- Already parsed