README.md: Fix link formatting

This change trivially modifies the link to the `mock_printer`
documentation to format correctly.

BUG=b:178649877
TEST=Gitiles preview

Change-Id: I221ce9f091041d77c9cfb6b7d8820c9bbc20c4cb
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/virtual-usb-printer/+/2847476
Reviewed-by: Piotr Pawliczek <pawliczek@chromium.org>
Tested-by: Kalvin Lee <kdlee@chromium.org>
Commit-Queue: Kalvin Lee <kdlee@chromium.org>
diff --git a/README.md b/README.md
index 61e2066..5df515a 100644
--- a/README.md
+++ b/README.md
@@ -47,7 +47,7 @@
 `virtual-usb-printer` can behave like
 *   a printer,
 *   an eSCL scanner, or
-*   a [scriptable mock printer][./mock_printer/README.md].
+*   a [scriptable mock printer](./mock_printer/README.md).
 ***
 
 Before the first use you have to load a required kernel module: