Fix keyseq specification in Bash

[akinomyoga]

The current way of setting the keyseq as bind "'\C-r': '...'" appears to work as expected, but this is accidental. This is not the format that Readline supports; the keyseq cannot be enclosed within single quotes. This is now working because of Readline's loose treatment of the keyseqs, but this will easily be broken with a slightly different key representation.

Readline's interpretation of '\C-r' is as follows:

1. This doesn't start with the double quote, so Readline considers a legacy specification for a single key.
2. Readline splits the string with hyphens to extract the unmodified key name: r'. This doesn't match any known key names such as ESC, DEL, etc., so Readline just picks up the first character r.
3. To obtain the set of modifiers, Readline searches for C- and M- against '\C-r'. Now C- matches.
4. As a result, Readline generates the key "C-" plus "r", which accidentally matches the intended one.

However, this doesn't work e.g. with a different representation of C-r: '\x12'. This time, the string doesn't contain a hyphen, so
Readline considers '\x12' is the full key name. It doesn't match any known key name, so Readline extracts the first character ' as the key, which is different from the intention. One can confirm this interpretation by the following commands:

$ bind "'\x12': 'hello'"; bind -s
"'": "hello"        # <-- We here expect "\C-r" but it's instead interpreted as a single-quote character
$ bind '"\C-r": "hello"'; bind -s
"\C-r": "hello"     # <-- "\C-r" works as expected.
"'": "hello"

In this PR, I suggest using the standard " instead of unsupported '. I modified it by reversing ' and ". The macro string (i.e., the righ-hand side of :) can actually be quoted by single quotes, but I also modified it to " because it is easier for quoting.

[cantino]

Thank you!