docs: update table of contents to match new heading

[MrEeeeet111]

This PR updates the table of contents in the staking-cli/README.md.

Changes:

• Updated the TOC entry from 'Getting Help' to 'Overview' to match the current document heading

[CLAassistant]

All committers have signed the CLA.

[MrEeeeet111]

@mrain @sveitser Hi! Just wanted to remind you about this PR. If you have some time, please take a look. Thank you!

[MrEeeeet111]

@jbearer @tbro @imabdulbasit @ss-es @pls148 @bfish713 @rob-maron @lukaszrzasik Hi! Just wanted to remind you about this PR. If you have some time, please take a look. Thank you!