globals.hh: fix broken link to nspawn example

The substitution included the `.` at the end of the URL, breaking it.

Author: ConnorBaker

src/libstore/include/nix/store/globals.hh

@@ -873,7 +873,7 @@ public:
             On Linux, Nix can run builds in a user namespace where they run as root (UID 0) and have 65,536 UIDs available.
             This is primarily useful for running containers such as `systemd-nspawn` inside a Nix build. For an example, see [`tests/systemd-nspawn/nix`][nspawn].
 
-            [nspawn]: https://github.com/NixOS/nix/blob/67bcb99700a0da1395fa063d7c6586740b304598/tests/systemd-nspawn.nix.
+            [nspawn]: https://github.com/NixOS/nix/blob/67bcb99700a0da1395fa063d7c6586740b304598/tests/systemd-nspawn.nix
 
             Included by default on Linux if the [`auto-allocate-uids`](#conf-auto-allocate-uids) setting is enabled.
         )",