docs/cmdline-opts: unify HTTP version style in --help output

[bagder]

$ curl -h all | grep -- --http

Now:

     --http0.9                                     Allow HTTP/0.9 responses
 -0, --http1.0                                     Use HTTP/1.0
     --http1.1                                     Use HTTP/1.1
     --http2                                       Use HTTP/2
     --http2-prior-knowledge                       Use HTTP/2 without HTTP/1.1 Upgrade
     --http3                                       Use HTTP/3
     --http3-only                                  Use HTTP/3 only

Before:

     --http0.9                            Allow HTTP 0.9 responses
 -0, --http1.0                            Use HTTP 1.0
     --http1.1                            Use HTTP 1.1
     --http2                              Use HTTP/2
     --http2-prior-knowledge              Use HTTP 2 without HTTP/1.1 Upgrade
     --http3                              Use HTTP v3
     --http3-only                         Use HTTP v3 only