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, 4 insertions, 4 deletions

diff --git a/docs/source/markdown/podman-image-prune.1.md b/docs/source/markdown/podman-image-prune.1.md
index 73024ffb8..16e6c9b03 100644
--- a/docs/source/markdown/podman-image-prune.1.md
+++ b/docs/source/markdown/podman-image-prune.1.md
@@ -14,11 +14,11 @@ does not have any containers based on it.
 The image prune command does not prune cache images that only use layers that are necessary for other images.
 
 ## OPTIONS
-#### **--all**, **-a**
+#### **\-\-all**, **-a**
 
 Remove dangling images and images that have no associated containers.
 
-#### **--filter**=*filters*
+#### **\-\-filter**=*filters*
 
 Provide filter values.
 
@@ -33,11 +33,11 @@ The until filter can be Unix timestamps, date formatted timestamps, or Go durati
 
 The label filter accepts two formats. One is the label=... (label=_key_ or label=_key=value_), which removes images with the specified labels. The other format is the label!=... (label!=_key_ or label!=_key=value_), which removes images without the specified labels.
 
-#### **--force**, **-f**
+#### **\-\-force**, **-f**
 
 Do not provide an interactive prompt for container removal.
 
-#### **--help**, **-h**
+#### **\-\-help**, **-h**
 
 Print usage statement