doc: fix list format in Developer's Certificate of Origin

This corresponds to the format at https://developercertificate.org/.

PR-URL: #37138
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Pooja D P <Pooja.D.P@ibm.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>

CONTRIBUTING.md

@@ -33,24 +33,24 @@ See [details on our policy on Code of Conduct](./doc/guides/contributing/code-of
 
 By making a contribution to this project, I certify that:
 
-* (a) The contribution was created in whole or in part by me and I
-  have the right to submit it under the open source license
-  indicated in the file; or
-
-* (b) The contribution is based upon previous work that, to the best
-  of my knowledge, is covered under an appropriate open source
-  license and I have the right under that license to submit that
-  work with modifications, whether created in whole or in part
-  by me, under the same open source license (unless I am
-  permitted to submit under a different license), as indicated
-  in the file; or
-
-* (c) The contribution was provided directly to me by some other
-  person who certified (a), (b) or (c) and I have not modified
-  it.
-
-* (d) I understand and agree that this project and the contribution
-  are public and that a record of the contribution (including all
-  personal information I submit with it, including my sign-off) is
-  maintained indefinitely and may be redistributed consistent with
-  this project or the open source license(s) involved.
+ (a) The contribution was created in whole or in part by me and I
+     have the right to submit it under the open source license
+     indicated in the file; or
+
+ (b) The contribution is based upon previous work that, to the best
+     of my knowledge, is covered under an appropriate open source
+     license and I have the right under that license to submit that
+     work with modifications, whether created in whole or in part
+     by me, under the same open source license (unless I am
+     permitted to submit under a different license), as indicated
+     in the file; or
+
+ (c) The contribution was provided directly to me by some other
+     person who certified (a), (b) or (c) and I have not modified
+     it.
+
+ (d) I understand and agree that this project and the contribution
+     are public and that a record of the contribution (including all
+     personal information I submit with it, including my sign-off) is
+     maintained indefinitely and may be redistributed consistent with
+     this project or the open source license(s) involved.