Fixed typo in API documentation.

Documentation/api.md

@@ -305,7 +305,7 @@ We get the index is outdated response, since we miss the 1000 events kept in etc
 {"errorCode":401,"message":"The event in requested index is outdated and cleared","cause":"the requested history has been cleared [1003/7]","index":2002}
 ```
 
-To start watch, frist we need to fetch the current state of key `/foo` and the etcdIndex.
+To start watch, first we need to fetch the current state of key `/foo` and the etcdIndex.
 ```sh
 curl 'http://127.0.0.1:2379/v2/keys/foo' -vv
 ```