Fix documentation of examples

[olebole]

The examples were often lazily formatted, with either completely being in a .nf/.fi (preformatted/code) section or completely outside, like for fgwrite:

2. Create an archive of the current directory and its subdirectories excluding any symbolic links. cl> fgwrite . ../zzd_arc.fits exclude=s checksum=yes The ckecksum option is set, so the keyword CHECKSUM, DATASUM and CHECKVER will be present in all unit headers.

This PR corrects this:

2. Create an archive of the current directory and its subdirectories excluding any symbolic links.

    cl> fgwrite . ../zzd_arc.fits exclude=s checksum=yes 
   

The ckecksum option is set, so the keyword CHECKSUM, DATASUM and CHECKVER will be present in all unit headers.