Clarify formatting of character 'l' in ch08-02-strings.md

[gregorgatej]

Updated a line in ch08-02-strings.md where the character l was originally italicized as _l_. In rendered Markdown, this can be easily confused with a slash '/'.

The update replaces _l_ with l (using backticks), to improve clarity.

Note: This change shares the same intent as commit 2f04f65 by @reinvantveer, which fixed a formatting issue for the same character, albeit with a different proposal ("l" instead of l).