build: fix HTML errors in nwfilter docs

build: fix HTML errors in nwfilter docs

A build on Ubuntu reported:

|| Generating formatnwfilter.html.tmp
/dados/develop/libvirt/docs/formatnwfilter.html.in|390| HTML parser error : Unexpected end tag : p
||     </p>
||         ^
/dados/develop/libvirt/docs/formatnwfilter.html.in|705| HTML parser error : Unexpected end tag : code
||          <td>End of range of valid source ports</code></td>
||                                                       ^
/dados/develop/libvirt/docs/formatnwfilter.html.in|710| HTML parser error : Unexpected end tag : code
||          <td>Start of range of valid destination ports</code></td>
||                                                              ^

* docs/formatnwfilter.html.in: Fix invalid HTML constructs.
Reported by Eduardo Otubo.