French translation typo in comments for slice syntax with omitted indexes

Translation typo in comments for slice syntax with omitted indexes:
"Omit the beginning" meant omit the beginning *of the list* (not the beginning of the range)
"Omit the end" meant omit the end *of the list* (not the end of the range)
Merge pull request #383 from C-Duv/python-fr.sliceTypoFix

[python/fr-fr] French translation typo in comments for slice syntax with omitted indexes