gh-38765: Fix time limitation in magma_free docstring
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Change time limitation from 20 to 120 in the documentation of
magma_free.
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> Calculations are restricted to 120 seconds.
http://magma.maths.usyd.edu.au/calc/
> <max_time>120</max_time>
http://magma.maths.usyd.edu.au/xml/calculator.xml?input=Factorization(15
)
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### :hourglass: Dependencies
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URL: https://github.com/sagemath/sage/pull/38765
Reported by: Eloi Torrents
Reviewer(s): Travis Scrimshaw, Vincent Macri
