docs: Use \ instead of \\ in examples (#7286)

Instead of informing the bell character (hex 07), the example was using
an escaped \ followed by x07.

    $ echo \\x07
    \x07
    $ echo \x07

    $ echo \x07 | od -a
    0000000 bel  nl
    0000002
    $

* docs: Use \u instead of \\u

Instead of informing the Unicode character 慡, this example was using an
escaped \ followed by u6161.

    $ echo \\u6161
    \u6161
    $ echo \u6161
    慡

Before:

    $ string escape --style=var 'a1 b2'\\u6161 | string unescape --style=var
    a1 b2\u6161

Now:

    $ string escape --style=var 'a1 b2'\u6161 | string unescape --style=var
    a1 b2慡

doc_src/cmds/echo.rst

@@ -64,7 +64,7 @@ Example
    > echo 'Hello World'
    Hello World
 
-   > echo -e 'Top\\nBottom'
+   > echo -e 'Top\nBottom'
    Top
    Bottom