Bump protobuf from 3.14.0 to 3.15.0 in /docs/examples/fork-process-model/flask-gunicorn (open-telemetry#2466)

Author: lzchen

docs/examples/fork-process-model/flask-gunicorn/requirements.txt

@@ -12,7 +12,7 @@ opentelemetry-instrumentation==0.18b0
 opentelemetry-instrumentation-flask==0.18b1
 opentelemetry-instrumentation-wsgi==0.18b1
 opentelemetry-sdk==0.18b0
-protobuf==3.14.0
+protobuf==3.15.0
 six==1.15.0
 thrift==0.13.0
 uWSGI==2.0.19.1