Fix Flask instrumentation doc link (#1216)

Author: trstringer

instrumentation/opentelemetry-instrumentation-flask/README.rst

@@ -61,6 +61,6 @@ Flask Request object reference: https://flask.palletsprojects.com/en/2.0.x/api/#
 References
 ----------
 
-* `OpenTelemetry Flask Instrumentation <https://opentelemetry-python-contrib.readthedocs.io/en/stable/instrumentation/flask/flask.html>`_
+* `OpenTelemetry Flask Instrumentation <https://opentelemetry-python-contrib.readthedocs.io/en/latest/instrumentation/flask/flask.html>`_
 * `OpenTelemetry Project <https://opentelemetry.io/>`_
 * `OpenTelemetry Python Examples <https://github.com/open-telemetry/opentelemetry-python/tree/main/docs/examples>`_