docs/esp8266/tutorial: Fix typo, "its" to "it's" in powerctrl.rst.

Author: mattdbr

docs/esp8266/tutorial/powerctrl.rst

@@ -22,7 +22,7 @@ processing power, at the expense of current consumption::
     160000000
 
 You can change to the higher frequency just while your code does the heavy
-processing and then change back when its finished.
+processing and then change back when it's finished.
 
 Deep-sleep mode
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