Fix long option format on docs.podman.io

Escape the two dashes, otherwise they are combined into one long dash.
I tested that this change is safe and still renders correctly on github
and with the man pages.

This commit also contains a small change to make it build locally.
Assuming you have the dependencies installed you can do:
```
cd docs
make html
```
Preview the html files in docs/build/html with
`python -m http.server 8000 --directory build/html`.

Fixes containers/podman.io#373

Signed-off-by: Paul Holzinger <paul.holzinger@web.de>

1 files changed, 5 insertions, 5 deletions

diff --git a/docs/source/markdown/podman-image-sign.1.md b/docs/source/markdown/podman-image-sign.1.md
index b9addc062..994578530 100644
--- a/docs/source/markdown/podman-image-sign.1.md
+++ b/docs/source/markdown/podman-image-sign.1.md
@@ -15,24 +15,24 @@ By default, the signature will be written into `/var/lib/containers/sigstore` fo
 
 ## OPTIONS
 
-#### **--help**, **-h**
+#### **\-\-help**, **-h**
 
 Print usage statement.
 
-#### **--all**, **-a**
+#### **\-\-all**, **-a**
 
 Sign all the manifests of the multi-architecture image (default false).
 
-#### **--cert-dir**=*path*
+#### **\-\-cert-dir**=*path*
 
 Use certificates at *path* (\*.crt, \*.cert, \*.key) to connect to the registry.
 Default certificates directory is _/etc/containers/certs.d_. (This option is not available with the remote Podman client)
 
-#### **--directory**, **-d**=*dir*
+#### **\-\-directory**, **-d**=*dir*
 
 Store the signatures in the specified directory.  Default: /var/lib/containers/sigstore
 
-#### **--sign-by**=*identity*
+#### **\-\-sign-by**=*identity*
 
 Override the default identity of the signature.