docs/manual: add sob line for patch series

The -s is missing in the documentation when teaching how to use 'git
format-patch' to automatically add the sob line.

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

docs/manual/contribute.txt

@@ -272,7 +272,7 @@ This can be easily handled with +git format-patch+ by using the option
 
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 $ git format-patch --subject-prefix "PATCH v4" \
-    -M -o outgoing origin/master
+    -M -s -o outgoing origin/master
 ---------------------
 
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