Always use $ as shell prompt in example

Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4359)

doc/man1/engine.pod

@@ -67,7 +67,7 @@ See the example below.
 
 To list all the commands available to a dynamic engine:
 
- % openssl engine -t -tt -vvvv dynamic
+ $ openssl engine -t -tt -vvvv dynamic
  (dynamic) Dynamic engine loading support
       [ unavailable ]
       SO_PATH: Specifies the path to the new ENGINE shared library
@@ -87,7 +87,7 @@ To list all the commands available to a dynamic engine:
 
 To list the capabilities of the I<rsax> engine:
 
- % openssl engine -c
+ $ openssl engine -c
  (rsax) RSAX engine support
   [RSA]
  (dynamic) Dynamic engine loading support