Document tab key to change tab

[Thra11]

Describe the bug
It took me a while to work out how to change to the "Log" tab, as the key to press, tab, isn't mentioned in the help.

To Reproduce
Steps to reproduce the behavior:

1. Open gitui
2. press 'h' to view the help

Expected behavior
The help should show the key to press to change tab, (Something like "Change tab [tab]") probably in the General section.

Desktop (please complete the following information):

• OS: NixOS Linux
• Version 0.2.1

[extrawurst]

ha very good point! Thanks @Thra11 for the hint. just wanted to release tags in the log (#47) but let me first fix that too