Issue #2346101 by realityloop: Fixed Alternate text should be called Alternative text.

core/modules/image/config/schema/image.data_types.schema.yml

core/modules/image/config/schema/image.schema.yml

core/modules/image/src/Plugin/Field/FieldType/ImageItem.php

...odules/image/src/Plugin/Field/FieldWidget/ImageWidget.php

core/modules/image/src/Tests/ImageFieldDisplayTest.php

core/modules/image/src/Tests/ImageFieldValidateTest.php

core/modules/image/templates/image-style.html.twig