[BEAM-9031] Fixes wrong Python example in Flink runner documentation

[ozturkberkay]

Current example throws: TypeError: Runner PipelineOptions() is not a PipelineRunner object or the name of a registered runner since the Pipeline implementation is as follows:

class Pipeline(object):
  def __init__(self, runner=None, options=None, argv=None):
    """Initialize a pipeline object.

RE: @ibzib

---

Lang	SDK	Apex	Dataflow	Flink	Gearpump	Samza	Spark
Go		---	---		---	---
Java
Python		---			---	---
XLang	---	---	---		---	---	---

Pre-Commit Tests Status (on master branch)

---	Java	Python	Go	Website
Non-portable
Portable	---		---	---

See .test-infra/jenkins/README for trigger phrase, status and link of all Jenkins jobs.

[iemejia]

LGTM. Thanks @ozturkberkay