manual: Add id to legalnotice.

Otherwise the generated HTML will have a different randomly generated
name id which generates spurious diffs.
diff --git a/manual.xml b/manual.xml
index b99e480..ea9fca2 100644
--- a/manual.xml
+++ b/manual.xml
@@ -27,7 +27,7 @@
    </author>
   </authorgroup>
 
-  <legalnotice>
+  <legalnotice id="legal">
 
   <para>This program, <computeroutput>bzip2</computeroutput>, the
   associated library <computeroutput>libbzip2</computeroutput>, and