Use ≡ for the hamburguer menu in the onboarding

reported by translator (thanks AO!):

In chrome/locale/en-US/browserOnboarding.properties we could use the char ≡ instead of = to refer to the hamburguer menu.

here a branch: https://dip.torproject.org/emmapeel/torbutton/commits/hamburguer

and the commit: https://dip.torproject.org/emmapeel/torbutton/commit/357dc2f1735e16252f6ea6e61b385182a2929f70

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