Mention the actual output type on an OutputMismatch error (#13355)

# Description

This improves the error when the determined output of a custom command
doesn't match the specified output type by adding the actual determined
output type.

# User-Facing Changes

Previous: `command doesn't output {0}`

New: `expected {0}, but command outputs {1}`

# Tests + Formatting
Passing.

# After Submitting
- [ ] release notes? (minor change, but helpful)

crates/nu-parser/src/type_check.rs

@@ -1055,7 +1055,11 @@ pub fn check_block_input_output(working_set: &StateWorkingSet, block: &Block) ->
                     .span
             };
 
-            output_errors.push(ParseError::OutputMismatch(output_type.clone(), span))
+            output_errors.push(ParseError::OutputMismatch(
+                output_type.clone(),
+                current_output_type.clone(),
+                span,
+            ))
         }
     }