Luhn: Update test of non-digit (#496)

* Luhn: Update test of non-digit

While implementing these tests for Rust I found that the final test was
not well written (my fault!)

exercism/rust#247

I added the alphabetical character to an already-invalid string. So the
test doesn't show anything useful.

Instead the test should show that a known-valid string is made invalid
by the inclusion of a non-digit

exercises/luhn/canonical-data.json

@@ -26,8 +26,8 @@
       "expected": false
     },
     {
-      "description": "strings that contain non-digits are not valid",
-      "input": "827a 1232 7352 0569",
+      "description": "valid strings with a non-digit added become invalid",
+      "input": "046a 454 286",
       "expected": false
     }
   ]