Fix wrong document formatting

Author: suiso67

docs/installation.rst

@@ -10,7 +10,7 @@ OS/distro packages
 The following packages are community-contributed and were up-to-date at the
 time of writing:
 
-- `ArchLinux <https://archlinux.org/packages/extra/any/vdirsyncer/>`_
+- `Arch Linux <https://archlinux.org/packages/extra/any/vdirsyncer/>`_
 - `Ubuntu and Debian, x86_64-only
   <https://packagecloud.io/pimutils/vdirsyncer>`_ (packages also exist
   in the official repositories but may be out of date)