Update and clarify grep regex usage in 05.Rmd

[lmanc]

• Corrected the initial note about the necessity of the -E option in grep. The original note suggested that without -E, grep would treat patterns as literal strings, which is wrong.
• Added brief explanations differentiating between Extended Regular Expressions (EREs) and Basic Regular Expressions (BREs). By default, grep operates with BRE, but with the -E flag, it switches to using ERE.
• Provided an example of how to achieve equivalent results using BRE with the -G flag.