Fix a long form DCO commit example

Closes #199

Signed-off-by: Lukáš Vlček <[email protected]>

CONTRIBUTING.md

@@ -84,7 +84,7 @@ Each commit must include a DCO which looks like this
 Signed-off-by: Jane Smith <[email protected]>
 ```
 
-You may type this line on your own when writing your commit messages. However, if your user.name and user.email are set in your git configs, you can use `-s` or `– – signoff` to add the `Signed-off-by` line to the end of the commit message.
+You may type this line on your own when writing your commit messages. However, if your user.name and user.email are set in your git configs, you can use `-s` or `--signoff` to add the `Signed-off-by` line to the end of the commit message.
 
 ## Finding contributions to work on
 Looking at the existing issues is a great way to find something to contribute on. As our projects, by default, use the default GitHub issue labels (enhancement/bug/duplicate/help wanted/invalid/question/wontfix), looking at any 'help wanted' issues is a great place to start.