[fi] Change the words used as translation for stage/staging area/etc.

The previously used word is based on the wrong meaning of "stage", not the one intended by Git.
Update 01-chapter3.markdown

update in a few paragraph at Basic Branching.
Merge pull request #885 from lanurmi/terminology-stage

[fi] Change the words used as translation for stage/staging area/etc.
Merge pull request #886 from tamami/patch-2

Update 01-chapter3.markdown