Change "wm" to "Margin"

This PR changes the "wm" mode line to "Margin" which is more consistent with other mode lines. (Namely, that they tend to be short words, with an initial cap.)

For example, at the moment, using org mode, I see this:

```
(Org wm Wrap)
```

With this change, I would see:

```
(Org Margin Wrap)
```