Apply review suggestions to README

Signed-off-by: JLBuenoLopez-eProsima <[email protected]>

README.md

@@ -59,7 +59,7 @@ This entails that any blocking call occurring during the write operation would b
 It is important to note that this mode typically yields higher throughput rates at lower latencies, since the notification and context switching between threads is not present.
 * `AUTO`: let Fast DDS select the publication mode. This implies using the publication mode set in the XML file or, failing that, the default value set in Fast DDS (which currently is set to `SYNCHRONOUS`).
 
-By default, `rmw_fastrtps` assumes this variable to be `ASYNCHRONOUS`, which is the publication mode in use if this environment variable is not set.
+If `RMW_FASTRTPS_PUBLICATION_MODE` is not set, then `rmw_fastrtps_cpp` and `rmw_fastrtps_dynamic_cpp` behave as if it were set to `ASYNCHRONOUS`.
 
 ## Example