docs: fix missing backslash in certtool shell example

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

docs/system/tls.rst

@@ -64,7 +64,7 @@ interactive prompts from certtool::
    cert_signing_key
    EOF
    # certtool --generate-self-signed \
-              --load-privkey ca-key.pem
+              --load-privkey ca-key.pem \
               --template ca.info \
               --outfile ca-cert.pem