Fix po4a conf file

This line was declared twice, which causes the build of i10n docs to
fail.

Author: steveklabnik

src/doc/po4a.conf

@@ -26,4 +26,3 @@
 [type: text] src/doc/intro.md $lang:doc/l10n/$lang/intro.md
 [type: text] src/doc/rust.md $lang:doc/l10n/$lang/rust.md
 [type: text] src/doc/rustdoc.md $lang:doc/l10n/$lang/rustdoc.md
-[type: text] src/doc/guide.md $lang:doc/l10n/$lang/guide.md