Fix example in string length docs

Technically the equivalence would be something like

	string length -q $str
	test -n (string join \n -- $str | string collect)

To handle when str has multiple empty strings;
but quoting is easier to remember and enough for most practical purposes.

doc_src/cmds/string-length.rst

@@ -34,6 +34,6 @@ Examples
     >_ set str foo
     >_ string length -q $str; echo $status
     0
-    # Equivalent to test -n $str
+    # Equivalent to test -n "$str"
 
 .. END EXAMPLES