Fix argument variable's name to follow the document convention

[shockyng]

https://github.com/eclipse-ee4j/jakartaee-tutorial/blob/ef768309be4c3838a55b0f2c1743f9ffacc2b1f8/src/main/asciidoc/webapp/webapp003.adoc?plain=1#LL125C44-L125C44

May I change variables name from user_name to userName to follow the document convention and make sense using "this" reference, for this one and the variables.