Fix typos on pt_BR/gittutorial.txt translation

With extra fixes from Thadeu and Carlos as well.

Signed-off-by: André Goddard Rosa <[email protected]>
Signed-off-by: Thadeu Lima de Souza Cascardo <[email protected]>
Signed-off-by: Carlos R. Mafra <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: andre-rosa