docs/differences/index_template: Use consistent heading casing.

And in our case, "consistent" is where each word in the heading is *not*
capitalized.

docs/differences/index_template.txt

-MicroPython Differences from CPython
+MicroPython differences from CPython
 ====================================
 
 The operations listed in this section produce conflicting results in MicroPython when compared to standard Python.