Fix broken links in `contrib/learn/python/learn/README.md`

This fix fixes broken links in `contrib/learn/python/learn/README.md`
by replacing g3doc with appropriate URLs, e.g.,
```
-    ([docs](../../../../g3doc/api_docs/python/contrib.learn.md#LinearClassifier))
+    ([docs](https://www.tensorflow.org/api_docs/python/tf/contrib/learn/LinearClassifier)

)
```

NOTE: Inside `contrib/learn/python/learn/README.md` there are still
several `g3doc` references. However the remaining links are actually
correct and are not broken.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>