Fix build warnings: add python_by_example label (#40)

- Added (python_by_example)= label to python_by_example.md to resolve undefined label references
- Fixes 2 of 3 build warnings in functions.md (lines 167, 208)
- Remaining warning for 'writing_good_code' document will be resolved when that lecture is translated

Author: mmcky

lectures/python_by_example.md

@@ -9,6 +9,7 @@ kernelspec:
   name: python3
 ---
 
+(python_by_example)=
 # یک مثال مقدماتی
 
 ## مقدمه