Fix grammar of menu item

[pingram3030]

1. headings is plural
2. heading's is possessive
3. headings' is plural possessive

The last is preferred in this case, but looks kinda luls, so singular tense should be used as:

• there may well only be a single heading.
• all other menu options default to single tense, for example Format -> Paragraph Styles.

A bit of a nit pick, as people can change it to be what ever they want, but I had a couple of minutes before lunch arrives, so I figured I'd make a minor fix :)