correct typo

src/main.rs

@@ -84,7 +84,7 @@ fn parse_config(conf: String, pool: &mut Vec<robot::Robot>) -> Result<world::Wor
             Some(raw) => raw,
         };
         if a.is_empty() {
-            return Err("Thi line should not be empty !");
+            return Err("This line should not be empty !");
         }
         let b = match lines.next() {
             None => return Err("This line should be the instruction !"),