README: Mention CONTRIBUTING not SubmittingPatches

The “SubmittingPatches” file is no more since commit
a76badf77a, but the READMEs were still
referencing it.
They now correctly point to “CONTRIBUTING.md”.

Signed-off-by: Paul Capron <paul@fragara.com>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>

README

@@ -36,7 +36,7 @@ https://www.libssh.org
 4* Contributing
 -_-_-_-_-_-_-_-_-_
 
-Please read the file 'SubmittingPatches' next to this README file. It explains
+Please read the file 'CONTRIBUTING.md' next to this README file. It explains
 our copyright policy and how you should send patches for upstream inclusion.
 
 Have fun and happy libssh hacking!

README.md

@@ -37,7 +37,7 @@ https://www.libssh.org
 
 # Contributing
 
-Please read the file 'SubmittingPatches' next to this README file. It explains
+Please read the file 'CONTRIBUTING.md' next to this README file. It explains
 our copyright policy and how you should send patches for upstream inclusion.
 
 Have fun and happy libssh hacking!