replace <outp> command with \outp in docs

Doxygen has been warning that `<outp>` and `</outp>` are not valid XML/HTML commands since commit cb6d5d76 on 20016-04-04. That's primarily because there is at present no way to tell Doxygen to recognize new XML/HTML tags. The actual errors look like this:

```
.../string.doxygen:187: warning: Unsupported xml/html tag </outp> found
```

I hate build errors since they a) cause needless concern, and b) make it harder to notice when I've introduced a new error. So switch from XML/C## style markup to Doxygen style markup for the "outp" annotation.

doc_src/index.hdr.in

@@ -551,22 +551,22 @@ Lists adjacent to other lists or strings are expanded as cartesian products:
 Examples:
 \fish{cli-dark}
 >_ echo {good,bad}" apples"
-<outp>good apples bad apples</outp>
+\outp{good apples bad apples}
 
 >_ set -l a x y z
 >_ set -l b 1 2 3
 
 >_ echo $a$b
-<outp>x1 y1 z1 x2 y2 z2 x3 y3 z3</outp>
+\outp{x1 y1 z1 x2 y2 z2 x3 y3 z3}
 
 >_ echo $a"-"$b
-<outp>x-1 y-1 z-1 x-2 y-2 z-2 x-3 y-3 z-3</outp>
+\outp{x-1 y-1 z-1 x-2 y-2 z-2 x-3 y-3 z-3}
 
 >_ echo {x,y,z}$b
-<outp>x1 y1 z1 x2 y2 z2 x3 y3 z3</outp>
+\outp{x1 y1 z1 x2 y2 z2 x3 y3 z3}
 
 >_ echo {$b}word
-<outp>1word 2word 3word</outp>
+\outp{1word 2word 3word}
 \endfish
 
 Be careful when you try to use braces to separate variable names from text. The dangers noted in the last example above can be avoided by wrapping the variable in double quotes instead of braces (`echo "$b"word`).