Fix doc references for nullary expressions.

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### Description
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Fix doc references for nullary expressions.

### Reference issue
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Fixes #2997.

### Additional information
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Closes #2997

See merge request libeigen/eigen!2054


(cherry picked from commit 04eb06b35435ab9e740eacdd08a7a101dd9c8237)

31a2702e Fix doc references for nullary expressions.

Co-authored-by: Antonio Sánchez <cantonios@google.com>
1 file changed