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>

docs/source/markdown/podman-completion.1.md

@@ -14,11 +14,11 @@ These script are used by the shell to provide suggestions and complete commands
 Usually these scripts are automatically installed via the package manager.
 
 ## OPTIONS
-#### **--file**, **-f**
+#### **\-\-file**, **-f**
 
 Write the generated output to file.
 
-#### **--no-desc**
+#### **\-\-no-desc**
 
 Do not provide description in the completions.