docs/esp8266/tutorial: Fix typo, change -> changed.
@@ -14,7 +14,7 @@ current frequency use::
>>> machine.freq()
80000000
-By default the CPU runs at 80MHz. It can be change to 160MHz if you need more
+By default the CPU runs at 80MHz. It can be changed to 160MHz if you need more
processing power, at the expense of current consumption::
>>> machine.freq(160000000)
