Update Falcon doc to not suggest a DATADOG_ envar (#2997)

Co-authored-by: Brett Langdon <[email protected]>

Author: ZStriker19

ddtrace/contrib/falcon/__init__.py

@@ -18,7 +18,7 @@
     app = falcon.API()
 
 To disable distributed tracing when using autopatching, set the
-``DATADOG_FALCON_DISTRIBUTED_TRACING`` environment variable to ``False``.
+``DD_FALCON_DISTRIBUTED_TRACING`` environment variable to ``False``.
 
 **Supported span hooks**