Update README.rst - Fix Markdown (#3156)

Fix broken Markdown.

Co-authored-by: Emídio Neto <9735060+emdneto@users.noreply.github.com>

Author: cyrille-leclerc

processor/opentelemetry-processor-baggage/README.rst

@@ -33,7 +33,8 @@ Do not put sensitive information in Baggage.
 To repeat: a consequence of adding data to Baggage is that the keys and
 values will appear in all outgoing HTTP headers from the application.
 
-## Usage
+Usage
+-----
 
 Add the span processor when configuring the tracer provider.