Fix globals.hh typo

src/libstore/globals.hh

@@ -554,7 +554,7 @@ public:
         R"(
           This option determines the maximum size of the `tmpfs` filesystem
           mounted on `/dev/shm` in Linux sandboxes. For the format, see the
-          description of the `size` option of `tmpfs` in mount8. The default
+          description of the `size` option of `tmpfs` in mount(8). The default
           is `50%`.
         )"};