Address spelling errors; fix minor Markdown lint issues

Ed Torresrequested to merge
gitlab-community/gitlab-org/gitlab:gitlab-issuetemplate-planspike-uxtheme into master

What does this MR do and why?

This MR addresses spelling errors and minor Markdown lint issues in two files:

.gitlab/issue_templates/Plan - Spike.md
.gitlab/issue_templates/UX Theme.md

References

#580754

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