Period lattice: fix documentation error

[user202729]

That's what the code appear to be doing.

The existing example in _compute_periods_real also show it:

sage:             sage: # needs sage.rings.number_field
....:             sage: x = polygen(ZZ, 'x')
....:             sage: K.<a> = NumberField(x^3 - 2)
....:             sage: E = EllipticCurve([0,1,0,a,a])
....:             sage: embs = K.embeddings(CC)
....:             sage: Ls = [E.period_lattice(e) for e in embs]
....:             sage: [L.is_real() for L in Ls]
....: 
[False, False, True]
sage:             sage: Ls[2]._compute_periods_real(100)
....: 
(3.8145297721785450936365098936,
 1.9072648860892725468182549468 + 1.3404778596244020196600112394*I)

Then do

sage: ω1, ω2 = _
sage: ω2/ω1
0.50000000000000000000000000000 + 0.35141365769412556448697319411*I
sage: ω1/ω2
1.3387183410189190672591156778 - 0.94088781767934015952275257142*I

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.

⌛ Dependencies

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[kwankyu]

Nice catch. LGTM.