[gemini] tweak the contrib page

I find it more readable with some empty lines here and there

site/contrib.gmi

@@ -15,7 +15,9 @@ To build the image:
 ```instructions to build the image
 # docker build -f contrib/Dockerfile -t gmid .
 ```
+
 and then run it with something along the lines of:
+
 ```instruction to run the image
 # docker run --rm -it -p 1965:1965      \
         -v gmid.conf:/etc/gmid.conf:ro  \
@@ -29,9 +31,11 @@ and then run it with something along the lines of:
 => https://git.omarpolo.com/gmid/tree/contrib/gmid contrib/gmid
 
 contrib/gmid is a sample service file for OpenBSD rc(8).  To install it:
+
 ```command to install the rc file
 # cp contrib/gmid /etc/rc.d
 ```
+
 then the usual ‘rcctl [start|stop|enable|restart] gmid’ are available.
 
 ## Systemd unit file
@@ -39,9 +43,11 @@ then the usual ‘rcctl [start|stop|enable|restart] gmid’ are available.
 => https://git.omarpolo.com/gmid/tree/contrib/gmid.service gmid.service
 
 A simple service file for systemd.  To install it
+
 ```instructions to install the system file
 # cp contrib/gmid.service /lib/systemd/system/gmid.service
 ```
+
 then the usual ‘systemctl [status|start|enable|stop] gmid’ commands can be used to manage the server.
 
 Some things to keep in mind:
@@ -63,12 +69,14 @@ Some things to keep in mind:
 => https://git.omarpolo.com/gmid/tree/contrib/vim contrib/vim
 
 contrib/vim contains a syntax highlighting for vim.  To install it, just copy the files to ~/.vim or to /usr/share/vim/vimfiles, e.g.:
+
 ```install the vim syntax highlight in your home
 $ mkdir -p ~/.vim
 $ cp -R contrib/vim/* ~/.vim/
 ```
 
 To enable Syntastic checker, add to your vimrc:
+
 ```
 let g:syntastic_gmid_checkers = ['gmid']
 ```