README: Fix MicroPython ESP8266 port link.

README.md

@@ -142,7 +142,7 @@ The extra -I and -L flags are not needed when using the standalone SDK.
 Subdirectory `examples/` contains some example application(s) which
 can be built with esp-open-sdk. If you are interested in real-world,
 full-fledged, advanced example of a project built using esp-open-sdk,
-check https://github.com/micropython/micropython/tree/master/esp8266 .
+check https://github.com/micropython/micropython/tree/master/ports/esp8266.
 
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