COUNTER_Robots_list.json: Remove anchors from okhttp

The okhttp library now uses version strings in its user agent so we
should not anchor the string with ^ and $. For example, I have seen
both of these recently:

    okhttp/3.11.0
    okhttp/3.10.0

We could add a "/\d" to match the version string, but I think that
the "okhttp" is unique enough by itself to not be confused with a
real human user agent.

COUNTER_Robots_list.json

@@ -815,8 +815,8 @@
     "last_changed": "2020-07-02"
   }, 
   {
-    "pattern": "^okhttp$",
-    "last_changed": "2018-02-15",
+    "pattern": "okhttp",
+    "last_changed": "2020-02-25",
     "description": "okhttp is a Java HTTP client library.",
     "url": "http://square.github.io/okhttp/"
   },