Don't use single-character string constants in the is_backward function

See https://github.com/Manishearth/rust-clippy/wiki#single_char_pattern for further info

Signed-off-by: Victor Porof <vporof@mozilla.com>

edn/src/symbols.rs

@@ -169,7 +169,7 @@ impl NamespacedKeyword {
     /// ```
     #[inline]
     pub fn is_backward(&self) -> bool {
-        self.name.starts_with("_")
+        self.name.starts_with('_')
     }
 
     /// Whether this `NamespacedKeyword` should be interpreted in forward order.