docs: fix example for proxies in daemon.json

commit c846428cb6 added proxies to the
example `daemon.json`, based on the implementation that was added in
427c7cc5f8.

However, a follow-up pull request changed the proxy-configuration in`daemon.json`
to nest the configuration in a "proxies" struct, and the documentation was
not updated accordingly; see:
101dafd049

This patch fixes the example.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

docs/reference/commandline/dockerd.md

@@ -1252,9 +1252,11 @@ This is a full example of the allowed configuration options on Linux:
   "fixed-cidr-v6": "",
   "group": "",
   "hosts": [],
-  "http-proxy": "http://proxy.example.com:80",
-  "https-proxy": "https://proxy.example.com:443",
-  "no-proxy": "*.test.example.com,.example.org",
+  "proxies": {
+    "http-proxy": "http://proxy.example.com:80",
+    "https-proxy": "https://proxy.example.com:443",
+    "no-proxy": "*.test.example.com,.example.org",
+  },
   "icc": false,
   "init": false,
   "init-path": "/usr/libexec/docker-init",