src/libexpr/eval.hh: fix typo

The option name is `allowed-uris`, not `allowed-uri`.

src/libexpr/eval.hh

@@ -741,7 +741,7 @@ struct EvalSettings : Config
           If set to `true`, the Nix evaluator will not allow access to any
           files outside of the Nix search path (as set via the `NIX_PATH`
           environment variable or the `-I` option), or to URIs outside of
-          `allowed-uri`. The default is `false`.
+          `allowed-uris`. The default is `false`.
         )"};
 
     Setting<bool> pureEval{this, false, "pure-eval",