gh-38561: Period lattice: fix documentation error
That's what the code appear to be doing.
The existing example in `_compute_periods_real` also show it:
```sage
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
sage: ω1, ω2 = _
sage: ω2/ω1
0.50000000000000000000000000000 + 0.35141365769412556448697319411*I
sage: ω1/ω2
1.3387183410189190672591156778 - 0.94088781767934015952275257142*I
```
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preview.
### :hourglass: Dependencies
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URL: https://github.com/sagemath/sage/pull/38561
Reported by: user202729
Reviewer(s): Kwankyu Lee
