servo: Update README.md to match the new test location

Source-Repo: https://github.com/servo/servo
Source-Revision: 6ccb7324687cfa8f07b891a8f1f2fc6e722594f8

servo/README.md

@@ -23,4 +23,4 @@ On Debian-based Linuxes:
     mkdir build && cd build
     ../configure
     make check && make
-    ./servo ../test.html
+    ./servo ../src/test/test.html