Fix clippy warnings for lifetime elision in csv examples

Add explicit lifetime parameter `<'_>` to Span type to fix
mismatched_lifetime_syntaxes clippy warnings.

Co-authored-by: tomtau <[email protected]>

Author: Copilot

examples/csv.rs

@@ -19,7 +19,7 @@ mod ast {
     use super::csv::Rule;
     use pest::Span;
 
-    fn span_into_str(span: Span) -> &str {
+    fn span_into_str(span: Span<'_>) -> &str {
         span.as_str()
     }