doc(vignettes): language corrections in Seine vignettes

- Proposed by @guillaume.thirel (#43 (comment 47385))

refs #43
2 jobs for 43-review-documentation-for-publication-on-cran in 6 minutes and 3 seconds (queued for 14 seconds)
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passed check_as_cran #211943
docker

00:06:02

passed check_not_cran #211942
docker

00:05:07