Doc fix misalignment and typos in c8y-configuration-plugin specs

[rina23q]

Proposed changes

The how-to-guide says

The child device agent needs to upload this file to thin-edge with an HTTP PUT request to the URL: http://{tedge-ip}:8000/tedge/file-transfer/{child-id}/c8y-configuration-plugin

whilst the specs was written as

Uploads this file to http://$TEDGE_HTTP/tedge/file-transfer/$CHILD_DEVICE_ID/config_snapshot/c8y-configuration-plugin with a PUT call.

It seems $TEDGE_HTTP/tedge/file-transfer/{child-id}/c8y-configuration-plugin is correct (without config_snapshot), so I fix this part in the specs.

Also, found some typos in the doc, fixing them as well.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (general improvements like code refactoring that doesn't explicitly fix a bug or add any new functionality)
• Documentation Update (if none of the other choices apply)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Paste Link to the issue

Checklist

• I have read the CONTRIBUTING doc
• I have signed the CLA (in all commits with git commit -s)
• I ran cargo fmt as mentioned in CODING_GUIDELINES
• I used cargo clippy as mentioned in CODING_GUIDELINES
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments