Use single quotes for single char

Address https://rust-lang.github.io/rust-clippy/master/index.html#single_char_pattern

Change-Id: If9868e827a03890bf81808565a9eca3cb72f5516

codegenerator/templates/rust_macros.j2

@@ -371,7 +371,7 @@ Some({{ val }})
     {{ dst_var }}.{{ param.remote_name }}(
         val.iter()
             .cloned()
-            .map(|x| Vec::from([x.split(",").collect()]))
+            .map(|x| Vec::from([x.split(',').collect()]))
             .collect::<Vec<_>>(),
         );
   {%- elif param["setter_type"] is defined %}