fix: correct doc for eclair config

In this commit we correct the doc for enabling `keysend` in eclair.
Note: `--features.keysend=optional` is only usefull in Polar
because of how eclair docker image is built.
See: https://github.com/jamaljsr/polar/blob/master/docker/eclair/docker-entrypoint.sh#L25-L35

Author: Abdulkbk

README.md

@@ -35,7 +35,7 @@ have permission to execute commands on. Note that the current version
 of the simulator uses keysend to execute payments, which must be enabled as follows:
 * LND: `--accept-keysend`
 * CLN: enabled by default
-* Eclair: `--features.keysend=optional`
+* Eclair: `-Declair.features.keysend=optional` (or `--features.keysend=optional` if you're using Polar)
 
 ## Getting Started