Fix git flags in documentation

[olantwin]

Update documentation to use the correct git flag: -a doesn't exist. -A does (and was probably meant), but its long version --all is IMHO more understandable.

[coveralls]

Pull Request Test Coverage Report for Build 1099

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-2.1%) to 75.082%

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Totals
Change from base Build 1088:	-2.1%
Covered Lines:	2757
Relevant Lines:	3672

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💛 - Coveralls

[teto]

hum I think the -a is supposed to be passed to git and the manpage for git commit does mention -a. Seems like it's valid.

[olantwin]

Could this depend on the git version? With git 2.21.0, the -a flag does not exist for git-add.

Edit: I think the flag is passed to git-add, when used with papis git add. When I tried the command, I got the corresponding git add error.

[teto]

was looking at the wrong manpage (git commit instead of git add). Sry LGTM