Fix code example in library-user-guide/adding-udfs #14404
Description
Describe the bug
The code example in library-user-guide/adding-udfs appears to use an older syntax, resulting in compilation errors.
To Reproduce
Execute the code provided at https://datafusion.apache.org/library-user-guide/adding-udfs.html#adding-a-scalar-udf-by-create-udf.
Expected behavior
No response
Additional context
No response