opentelemetry-instrumentation: fix RST syntax of README (#3362)

Co-authored-by: Emídio Neto <[email protected]>

Author: xrmx

Diff for: opentelemetry-instrumentation/README.rst

@@ -131,7 +131,7 @@ The above command will configure the global trace provider to use the Random IDs
 pass ``--port=3000`` to ``flask run``.
 
 Programmatic Auto-instrumentation
---------------------
+---------------------------------
 
 ::