Fix a typo “an”→“a”

src/helpers.rs

@@ -78,7 +78,7 @@ fn eval_windows(&mut self, module: &str, name: &str) -> InterpResult<'tcx, Scala
         self.eval_context_mut().eval_path_scalar(&["std", "sys", "windows", module, name])
     }
 
-    /// Helper function to get a `windows` constant as an `u64`.
+    /// Helper function to get a `windows` constant as a `u64`.
     fn eval_windows_u64(&mut self, module: &str, name: &str) -> InterpResult<'tcx, u64> {
         // TODO: Cache the result.
         self.eval_windows(module, name)?.to_u64()