Fix axis-title-standoff.json yaxis title text

Claimed to have standoff 8 although it has standoff 0.

Author: my-tien

test/image/mocks/axis-title-standoff.json

@@ -26,7 +26,7 @@
     },
     "yaxis": {
       "title": {
-        "text": "Y<br>Axis (standoff:8)",
+        "text": "Y<br>Axis (standoff:0)",
         "standoff": 0,
         "font": {"size": 8}
       },