Flex and Foundations: add alt text for accessibility.

[Amandasilvbr]

Complete the following REQUIRED checkboxes:

• I have thoroughly read and understand The Odin Project Contributing Guide
• The title of this issue follows the location for request: brief description of request format, e.g. Foundations exercises: Add exercise for XYZ

The following checkbox is OPTIONAL:

• I would like to be assigned this issue to work on it

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1. Description of the Feature Request:

Four html files were missing the alt attribute in the img tag. This compromises accessibility. In addition, the use of alternative texts is considered a good practice and should be encouraged whenever appropriate.

2. Acceptance Criteria:

• Add alternative texts where missing