Tag alias: mechanical_arm -> mechanical_arms

Or vice versa. They mean the same thing anyway.

There are cases where there's only one mechanical arm, which would make the alias invalid.

I say we alias mechanical_arms -> mechanical_arm.
We usually alias plural -> singular unless it would be unnatural for there to be only one of something (which isn't the case here, as there are plenty like post #919950).

And if we have an significant distribution between singular and plural we don't alias it at all (e.g. horn:horns, hand_on_hip:hands_on_hips, etc.).

Well, if those tags are meant to represent plural vs singular, then they probably need wikis stating it and a good cleanup. mechanical_arm has plenty of posts with 2+ arms, even among the first ones.

Not sure we need to distinguish singular and plural for a tag like this.