chore: Fix snakecase convention in errors6.rs

Exercise errors6.rs prompts the user to add a method named `from_parseint`. This commit changes the method name to the corrected snakecase format, `from_parse_int`.

exercises/13_error_handling/errors6.rs

@@ -25,7 +25,7 @@ impl ParsePosNonzeroError {
     }
 
     // TODO: Add another error conversion function here.
-    // fn from_parseint(???) -> Self { ??? }
+    // fn from_parse_int(???) -> Self { ??? }
 }
 
 #[derive(PartialEq, Debug)]