Grammar fix

### What is this PR for?
This is a grammar fix for the README.

### What type of PR is it?
Improvement

### Todos
None

### What is the Jira issue?
I haven't opened a Jira issue, hopefully this is a small enough fix that it doesn't require one.

### How should this be tested?
N/A
### Screenshots (if appropriate)
None
### Questions:
None

Author: fredster33 <[email protected]>

Closes apache#4073 from fredster33/patch-1 and squashes the following commits:

f74f316 [fredster33] Grammar fix

README.md

@@ -10,7 +10,7 @@
 
 **Zeppelin**, a web-based notebook that enables interactive data analytics. You can make beautiful data-driven, interactive and collaborative documents with SQL, Scala and more.
 
-Core feature:
+Core features:
    * Web based notebook style editor.
    * Built-in Apache Spark support