nixos/modprobe: fix typo in boot.modprobeConfig.enable documentation

This typo was introduced in f4d8e64bed05489cdb2af880644f1da4d0fe4eef.
Also remove the dot at the end of the sentence since otherwise there would be two dots in the documentation.

nixos/modules/system/boot/modprobe.nix

···
   ###### interface
 
   options = {
-    boot.modprobeConfig.enable = mkEnableOption (lib.mdDoc "modprobe config. This is useful for systemds like containers which do not require a kernel.") // {
+    boot.modprobeConfig.enable = mkEnableOption (lib.mdDoc "modprobe config. This is useful for systems like containers which do not require a kernel") // {
       default = true;
     };