[SPARK-8639] [DOCS] Fixed Minor Typos in Documentation

Ticket: [SPARK-8639](https://issues.apache.org/jira/browse/SPARK-8639)

fixed minor typos in docs/README.md and docs/api.md

Author: Rosstin <asterazul@gmail.com>

Closes apache#7046 from Rosstin/SPARK-8639 and squashes the following commits:

6c18058 [Rosstin] fixed minor typos in docs/README.md and docs/api.md

Author: Rosstin

docs/README.md

@@ -28,7 +28,7 @@ in some cases:
     $ sudo gem install jekyll
     $ sudo gem install jekyll-redirect-from
 
-Execute `jekyll` from the `docs/` directory. Compiling the site with Jekyll will create a directory
+Execute `jekyll build` from the `docs/` directory to compile the site. Compiling the site with Jekyll will create a directory
 called `_site` containing index.html as well as the rest of the compiled files.
 
 You can modify the default Jekyll build as follows:

docs/api.md

@@ -3,7 +3,7 @@ layout: global
 title: Spark API Documentation
 ---
 
-Here you can API docs for Spark and its submodules.
+Here you can read API docs for Spark and its submodules.
 
 - [Spark Scala API (Scaladoc)](api/scala/index.html)
 - [Spark Java API (Javadoc)](api/java/index.html)