doc: sign release artifacts in armor mode

Release guide steps to artifacts signing defaults to binary
signatures, while producing .asc files.
This commit changes to armored signatures, also matching appc
requirements.

Fixes #5594

Documentation/dev-internal/release.md

@@ -66,7 +66,7 @@ The following commands are used for public release sign:
 
 ```
 cd release
-for i in etcd-*{.zip,.tar.gz}; do gpg2 --default-key $SUBKEYID --output ${i}.asc --detach-sign ${i}; done
+for i in etcd-*{.zip,.tar.gz}; do gpg2 --default-key $SUBKEYID --armor --output ${i}.asc --detach-sign ${i}; done
 for i in etcd-*{.zip,.tar.gz}; do gpg2 --verify ${i}.asc ${i}; done
 ```