Fix Vale issues for administration/auth/ldap/index.md

The following issues occurred when we ran a linting tool called "Vale" against this Markdown page in the GitLab documentation. The first number on each line indicates the line number where the error occurred.

To fix the issues, open a merge request for the markdown file, which you can find in the /doc directory. When you open the MR, a bot will automatically ping the appropriate GitLab technical writer. They will work with you to merge the changes.

If you feel any of these warnings are incorrect, please create an issue with the tw-test label so we can improve our Vale tests.

On the following lines, please replace need to with must. Thank you!

77	If possible, use "must" instead of "need to".
172	If possible, use "must" instead of "need to".
190	If possible, use "must" instead of "need to".
368	If possible, use "must" instead of "need to".
595	If possible, use "must" instead of "need to".
638	If possible, use "must" instead of "need to".
705	If possible, use "must" instead of "need to".