esp8266/README.md: Update build instruction to reflect new ports dir.

EddieParis · dpgeorge · commit 359d2bdf8450 · 2018-02-15T11:14:52.000+11:00
diff --git a/ports/esp8266/README.md b/ports/esp8266/README.md
@@ -49,7 +49,7 @@ $ make -C mpy-cross
 
 Then, to build MicroPython for the ESP8266, just run:
 ```bash
-$ cd esp8266
+$ cd ports/esp8266
 $ make axtls
 $ make
 ```
