Docs: Fix example with wrong key in Embedding Image with dict format

[DutchGerman]

In the docs on the page Embedding Image with dict format the following example is given:

gmail.send(
    subject='An image',
    receivers=['[email protected]'],
    html="""
        <h1>This is an image:</h1>
        {{ myimage }}
    """,
    body_images={
        'myimage': {
            'myimage': data_as_bytes,
            'subtype': 'png'
        }
    }
)

Yet the dict expects content instead of myimages as seen in the tests

red-mail/redmail/test/email/test_inline_media.py

Lines 89 to 92 in cf0396c

	'my_image': {
	"content": img_bytes,
	'subtype': 'jpg'
	}