[xmlparser] Use TextPositions own fmt

TextPosition starts counting line numbers at 0. There is a ready made
implementation of `fmt::Display` that accounts for this when showing a
TextPosition to a human.
Use this implementation instead of manually showing the zero-indexed
line number.

src/xmlparser.rs

@@ -38,10 +38,9 @@ impl ErrorEmitter {
     pub fn emit(&self, message: &str, position: TextPosition) -> String {
         let enriched = match self.path {
             Some(ref path) => format!(
-                "{} at line {}:{}: {}",
+                "{} at line {}: {}",
                 path.display(),
-                position.row,
-                position.column,
+                position,
                 message
             ),
             None => format!("{} {}", position, message),