Fix example of Pi calculation

Typo fix: `slidng` => `sliding`

Author: herwinw

learning/J_EXAMPLES.md

@@ -1,9 +1,9 @@
 **2021-01-15**<br>
 Pi Calculation:
 ```ijs
-slidng =: <;._3
-chunk  =: 3 : '(2*i.((#y) % 2)) { 2 sliding y'
-pi     =: 3 : '+/ > 8 % each */ each chunk 1 + 2 * i.y'
+sliding =: <;._3
+chunk   =: 3 : '(2*i.((#y) % 2)) { 2 sliding y'
+pi      =: 3 : '+/ > 8 % each */ each chunk 1 + 2 * i.y'
 ```
 **2021-01-17**<br>
 Pi Calculation (revised from above because we learned things):