fix a typo in the apt.conf manpage spotted by lintian

W: apt: spelling-error-in-manpage usr/share/man/man5/apt.conf.5.gz enviroment environment

doc/apt.conf.5.xml

@@ -408,7 +408,7 @@ DPkg::Pre-Install-Pkgs {"/usr/sbin/dpkg-preconfigure --apt";};
      files for every Language - especially the long Languagecodes are rare, so please
      inform you which ones are available before you set here impossible values.</para>
      <para>The default list includes "environment" and "en". "<literal>environment</literal>" has a special meaning here:
-     It will be replaced at runtime with the languagecodes extracted from the <literal>LC_MESSAGES</literal> enviroment variable.
+     It will be replaced at runtime with the languagecodes extracted from the <literal>LC_MESSAGES</literal> environment variable.
      It will also ensure that these codes are not included twice in the list. If <literal>LC_MESSAGES</literal>
      is set to "C" only the <filename>Translation-en</filename> file (if available) will be used.
      To force apt to use no Translation file use the setting <literal>Acquire::Languages=none</literal>. "<literal>none</literal>"