Added alt text to user image

[danhogan]

Description

Added alt text to user image in header component

Pre-Submission Checklist

By submitting this pull request, I have verified the following:

• Added descriptive name to pull request.

Your PR should NOT be called Update index.md, since it does not help the maintainer understand what has been changed.

Good Guide PR title examples:

fix(guide): Git - edit Git Commit article
feat(guide): PHP - create PHP section and add Data Structures article

Good Coding challenge PR title examples:

fix(coding): add tests to Basic HTML challenge
fix(coding): update instructions to CSS Grid challenge

This change is for the Learning platform or Coding Challenges, and:

• I tested my changes by running freeCodeCamp locally.
• All new and existing tests pass the command npm test.

This change is for Guide Articles, and:

• I confirm no plagiarized, duplicate, or repetitive content that has been directly copied from another source.
• I am adding only https links to external sources.
• I am not adding short links instead of full URLs to webpages.

Common requirements

• This pull request targets the master branch of freeCodeCamp.
• Branch starts with either fix/, feature/, or translate/
  (e.g. fix/signin-issue)
• I reviewed necessary formatting guidelines in CONTRIBUTING.md.

Issues Closures

• Addressed currently open issue. (replace XXXXX with an issue no in next line)

Closes #XXXXX

(Optional) Additional Context and Screenshots:

[raisedadead]

✨ ✨ LGTM. ✨ ✨

Thanks @danhogan

Happy Contributing!