Show missing backslash in manual

[zstadler]

The 1.6 manual about sub(regex;tostring) currently says:

a reference to a captured variable named "x" would take the form: "(.x)".

Using "(.x)" produces this string literally rather than the captured variable. Instead, the manual should say:

a reference to a captured variable named "x" would take the form: "\(.x)".

This PR may also need to be back-ported to earlier versions of the jq manual as well as ported to the development version of the jq manual.

[zstadler]

As far as I can tell, the CI failure has nothing to do with the contents of this PR

C:\msys64\usr\bin\bash -lc "pacman-key --verify msys2-keyring-r21.b39fb11-1-any.pkg.tar.xz.sig"
==> Checking msys2-keyring-r21.b39fb11-1-any.pkg.tar.xz.sig...
gpg: no valid OpenPGP data found.
gpg: the signature could not be verified.
Please remember that the signature file (.sig or .asc)
should be the first file given on the command line.
==> ERROR: The signature identified by msys2-keyring-r21.b39fb11-1-any.pkg.tar.xz.sig could not be verified.
Command exited with code 1
cat config.log
cat: config.log: No such file or directory
Command exited with code 1

[zstadler]

@stedolan, @nicowilliams

Please look at this documentation fix PR.

The failure in the CI check is unrelated to the change in the README.md file,

[wader]

Fix LGTM but have no access to merge