[FW][FIX] rpc: fix code examples for tutorial and service

[fw-bot]

I am a newbie from April and was following the "Discover the web framework" guide (https://www.odoo.com/documentation/18.0/developer/tutorials/discover_js_framework/02_build_a_dashboard.html#call-the-server-add-some-statistics) and saw that the link and example provided were outdated.

For example, it was like this:

setup() {
      this.rpc = useService("rpc");
      onWillStart(async () => {
         const result = await this.rpc("/my/controller", {a: 1, b: 2});
         // ...
      });
}

Which is now

import { rpc } from "@web/core/network/rpc";
// ... 

setup() {
      onWillStart(async () => {
            const syncResult = await rpc("/my/controller", {a: 1, b: 2});
      })
      // ...
}

I also changed the code example inside the Reference for the RPC service as it was also out of date. There might be a need to move this to another category as it might not be a Service anymore, I wasn't able to determine is it would be best or not (or which one would be best).

Forward-Port-Of: #13151

[robodoo]

[fw-bot]

This PR targets saas-18.3 and is part of the forward-port chain. Further PRs will be created up to master.

More info at https://github.com/odoo/odoo/wiki/Mergebot#forward-port

[robodoo]

@leclerc-leo @fdardenne staging failed: ci/runbot on 3ff98c0bd2e3958513a88b9a7005b0f530aa0117 (view more at https://runbot.odoo.com/runbot/build/79904648)

[fdardenne]

@robodoo retry