Grammar fix

git-svn-id: http://svn.linuxfromscratch.org/LFS/trunk/BOOK@10616 4aa44e1e-78dd-0310-a6d2-fbcd4c07a689

prologue/why.xml

@@ -544,7 +544,7 @@
 
         <para>This package contains programs for compressing and decompressing
         files.  It provides the highest compression generally available
-        and is useful for decompressing packages XZ or LZMA format.</para>
+        and is useful for decompressing packages in XZ or LZMA format.</para>
       </listitem>
 
       <listitem>