docs(aws-sdk-js-v3): corrected Content-Type presigning examples

[BoogeyMan24]

Summary

The documentation was stating that:

// Generate presigned URL for writing (PUT)
// Specify ContentType to restrict uploads to a specific file type
const putUrl = await getSignedUrl(
	S3,
	new PutObjectCommand({
		Bucket: "my-bucket",
		Key: "image.png",
		ContentType: "image/png",
	}),
	{ expiresIn: 3600 },
);

However, ContentType header is NOT signed by default and signableHeaders must be added for the header to be signed:

// Generate presigned URL for writing (PUT)
// Specify ContentType and add it to signableHeaders to restrict uploads to a specific file type
const putUrl = await getSignedUrl(
	S3,
	new PutObjectCommand({
		Bucket: "my-bucket",
		Key: "image.png",
		ContentType: "image/png",
	}),
	{ 
		expiresIn: 3600,
		signableHeaders: new Set(["content-type"]),
	},
);

This PR adds the required signableHeaders to the examples in the documentation so that Content-Type is signed.
Source: https://docs.aws.amazon.com/AWSJavaScriptSDK/v3/latest/Package/-aws-sdk-s3-request-presigner/

Screenshots (optional)

none

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