Change documentation about maintainer alumnus status field

[kytrinyx]

The linter chokes if given a string here. I'm not sure if the linter changed, or if this never worked in the first place as documented.

[iHiD]

We do accept strings in the website code, but I prefer that we don't. I believe we decided to allow it in case there was someone who was uncomfortable with the word alumnus, but I suggest we just merge this for now, and address it if we ever hit that scenario.