Update Github issue template (#14268)

Author: TomAugspurger

.github/ISSUE_TEMPLATE.md

@@ -1,6 +1,15 @@
-#### Code Sample, a copy-pastable example if possible
+#### A small, complete example of the issue
+
+```python
+# Your code here
+
+```
 
 #### Expected Output
 
-#### output of ``pd.show_versions()``
+#### Output of ``pd.show_versions()``
+
+<details>
+# Paste the output here
 
+</details>