fix: incorrectly mapped toggleDevTools in custom menu fiddle

[Kishanjay]

Description of Change

Fixes the toggleDevTools call in custom menu fiddle.

It has been wrongly mapped to the BrowserWindow object instead of the webContents object.

Checklist

• PR description included and stakeholders cc'd
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• relevant documentation, tutorials, templates and examples are changed or added
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Release Notes

Notes: Fixed toggleDevTools() code example in documentation.

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[ckerr]

Agreed.

History / Trivia: the BrowserWindow call probably works as written, since it has a helper function:

BrowserWindow.prototype.toggleDevTools = function () {
  return this.webContents.toggleDevTools();
};

But according to 80e5007, this is a "Deprecated / undocumented BrowserWindow [method]" so we shouldn't be using it in our docs / examples 😸

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[release-clerk]

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