fix commit amend dash

Signed-off-by: Alex Pop <[email protected]>

CONTRIBUTING.md

@@ -96,7 +96,7 @@ The DCO requires a sign-off message in the following format appear on each commi
 Signed-off-by: Julia Child <[email protected]>
 ```
 
-The DCO text can either be manually added to your commit body, or you can add either **-s** or **--signoff** to your usual git commit commands. If you forget to add the sign-off you can also amend a previous commit with the sign-off by running **git commit –-amend -s**. If you've pushed your changes to Github already you'll need to force push your branch after this with **git push -f**.
+The DCO text can either be manually added to your commit body, or you can add either **-s** or **--signoff** to your usual git commit commands. If you forget to add the sign-off you can also amend a previous commit with the sign-off by running **git commit --amend -s**. If you've pushed your changes to Github already you'll need to force push your branch after this with **git push -f**.
 
 ### Obvious Fix Policy