[MINOR] Modify the description of POST /sessions/{sessionId}/completion

## What changes were proposed in this pull request?

Just modified a description of POST /sessions/{sessionId}/completion in the api-doc.

## How was this patch tested?

Since the change is quite small, I didn't test the patch. If I have an instruction, I will follow it.

Author: Shingo Furuyama <sfuruyam@yahoo-corp.jp>

Closes #285 from marblejenka/mod-doc-completion.

Author: sfuruyam

docs/rest-api.md

@@ -312,7 +312,7 @@ Cancel the specified statement in this session.
 
 ### POST /sessions/{sessionId}/completion
 
-Runs a statement in a session.
+Returns code completion candidates for the specified code in the session.
 
 #### Request Body