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-events.1.md

@@ -70,16 +70,16 @@ The *volume* type will report the following statuses:
 
 ## OPTIONS
 
-#### **--help**
+#### **\-\-help**
 
 Print usage statement.
 
-#### **--format**
+#### **\-\-format**
 
 Format the output to JSON Lines or using the given Go template.
 
 
-#### **--filter**=*filter*
+#### **\-\-filter**=*filter*
 
 Filter events that are displayed.  They must be in the format of "filter=value".  The following
 filters are supported:
@@ -93,12 +93,12 @@ filters are supported:
 
 In the case where an ID is used, the ID may be in its full or shortened form.
 
-#### **--since**=*timestamp*
+#### **\-\-since**=*timestamp*
 
 Show all events created since the given timestamp
 
 
-#### **--until**=*timestamp*
+#### **\-\-until**=*timestamp*
 
 Show all events created until the given timestamp