[SPARK-6596] fix the instruction on building scaladoc

In README.md under docs/ directory, it says that

> You can build just the Spark scaladoc by running build/sbt doc from the SPARK_PROJECT_ROOT directory.

I guess the right approach is build/sbt unidoc

Author: CodingCat <zhunansjtu@gmail.com>

Closes #5253 from CodingCat/SPARK-6596 and squashes the following commits:

af379ed [CodingCat] fix the instruction on building scaladoc

docs/README.md

@@ -60,7 +60,7 @@ We use Sphinx to generate Python API docs, so you will need to install it by run
 
 ## API Docs (Scaladoc and Sphinx)
 
-You can build just the Spark scaladoc by running `build/sbt doc` from the SPARK_PROJECT_ROOT directory.
+You can build just the Spark scaladoc by running `build/sbt unidoc` from the SPARK_PROJECT_ROOT directory.
 
 Similarly, you can build just the PySpark docs by running `make html` from the
 SPARK_PROJECT_ROOT/python/docs directory. Documentation is only generated for classes that are listed as
@@ -68,7 +68,7 @@ public in `__init__.py`.
 
 When you run `jekyll` in the `docs` directory, it will also copy over the scaladoc for the various
 Spark subprojects into the `docs` directory (and then also into the `_site` directory). We use a
-jekyll plugin to run `build/sbt doc` before building the site so if you haven't run it (recently) it
+jekyll plugin to run `build/sbt unidoc` before building the site so if you haven't run it (recently) it
 may take some time as it generates all of the scaladoc.  The jekyll plugin also generates the
 PySpark docs [Sphinx](http://sphinx-doc.org/).