fix: middleware example to correctly return yield result

[S0nee]

In the Middleware code block, the example in README was incorrect:

class MyMiddleware
  def call(metadata)
    puts "Before execution"
    yield
    puts "After execution"
    result
  end
end

Why this is wrong:

• The variable result is used, but it is not assigned anywhere.
• The method does not return a yield result.

Correction:

class MyMiddleware
  def call(metadata)
    puts "Before execution"
    result = yield
    puts "After execution"
    result
  end
end

Now, an example:

• Stores the result of executing the block in result.
• Explicitly returns the result of the block, allowing the middleware chain to pass values correctly.