Manual: fix the example

Things managed to work once but for rather arbitrary reasons.

pdf-simple-sign.adoc

@@ -50,10 +50,11 @@ Examples
 Create a self-signed certificate, make a document containing the current date,
 sign it and verify the attached signature:
 
- $ openssl req -newkey rsa:2048 -subj "/CN=Test" -nodes
-   -keyout key.pem -x509 -out cert.pem 2>/dev/null
+ $ openssl req -newkey rsa:2048 -subj /CN=Test -nodes \
+   -keyout key.pem -x509 -addext keyUsage=digitalSignature \
+   -out cert.pem 2>/dev/null
  $ openssl pkcs12 -inkey key.pem -in cert.pem \
-   -export -passout pass:test -out key-cert.p12
+   -export -passout pass: -out key-pair.p12
  $ date | groff -T pdf > test.pdf
  $ pdf-simple-sign test.pdf test.signed.pdf key-pair.p12 ""
  $ pdfsig test.signed.pdf