Update link to crc_v3.txt.

Ross William's website is still up, but the section containing
the CRC introduction is no longer available. Instead link to
a copy on zlib.org which hopefully will remain available.

Also remove the mailto: link. Email is not one of the included
contact links on the author's site, so it seems rude to continue
to publish it here.

Signed-off-by: Mark Harris <mark.hsj@gmail.com>

doc/framing.html

@@ -366,10 +366,9 @@ entire header (with the CRC field in the header set to zero) and then
 continued over the page. The CRC field is then filled with the
 computed value.</p>
 
-<p>(A thorough discussion of CRC algorithms can be found in <a
-href="http://www.ross.net/crc/download/crc_v3.txt">"A
-Painless Guide to CRC Error Detection Algorithms"</a> by Ross
-Williams <a href="mailto:ross@ross.net">ross@ross.net</a>.)</p>
+<p>(A thorough discussion of CRC algorithms can be found in
+<a href="https://zlib.net/crc_v3.txt">"A Painless Guide to CRC Error Detection Algorithms"</a>
+by Ross Williams.)</p>
 
 <pre><tt>
  byte value