Making the parser error explicit (#2121)

If a user typed the error as it was originally it wouldn't trigger the expected parsing error.

With this modification now the error message, appears as it is expected in the document.

Author: elviejo79

book/thinking_in_nu.md

@@ -270,8 +270,8 @@ The following examples use the [`source` command](/commands/docs/source.md), but
 Consider this scenario:
 
 ```nu
-"print Hello" | save output.nu
-source output.nu
+("print Hello" | save output.nu;
+source output.nu)
 # => Error: nu::parser::sourced_file_not_found
 # =>
 # =>   × File not found