Fix signing command in contribution guide (#943)

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### Description

The correct parameter name is `--signoff` as per the official docs:
https://git-scm.com/docs/git-commit#Documentation/git-commit.txt--s
Otherwise, it is frustrating for occasional or first-time contributors
to be told to use parameters which the standard tools do not understand.

Additionally fixes a casing typo I stumbled upon when reading the
document.

Resolves or fixes issue: none

### AI Tool Disclosure

- [x] My contribution does not include any AI-generated content
- [ ] My contribution includes AI-generated content, as disclosed below:
  - AI Tools: `[e.g. GitHub CoPilot, ChatGPT, JetBrains Junie etc.]`
- LLMs and versions: `[e.g. GPT-4.1, Claude Haiku 4.5, Gemini 2.5 Pro
etc.]`
- Prompts: `[Summarize the key prompts or instructions given to the AI
tools]`

### Affirmation

- [x] My code follows the
[CONTRIBUTING.md](https://github.com/CycloneDX/cyclonedx-python-lib/blob/main/CONTRIBUTING.md)
guidelines

Signed-off-by: stefan6419846 <96178532+stefan6419846@users.noreply.github.com>

Author: stefan6419846

CONTRIBUTING.md

@@ -21,7 +21,7 @@ poetry install --all-extras
 
 ## Code style
 
-THis project loves latest python features.  
+This project loves latest python features.  
 This project loves sorted imports.  
 This project uses [PEP8] Style Guide for Python Code.  
 
@@ -67,7 +67,7 @@ Please sign off your commits, to show that you agree to publish your changes und
 , and to indicate agreement with [Developer Certificate of Origin (DCO)](https://developercertificate.org/).
 
 ```shell
-git commit --signed-off ...
+git commit --signoff ...
 ```
 
 ## Pre-commit hooks