Fix markup in ERC manual

* doc/misc/erc.texi (SASL): Use @table instead of simulating it
with @indentedblock.  Fix markup.  (Bug#59815)

doc/misc/erc.texi

@@ -983,20 +983,17 @@ explore all custom options pertaining to your chosen mechanism.
 
 @defopt erc-sasl-mechanism
 The name of an SASL subprotocol type as a @emph{lowercase} symbol.
+The value can be one of the following:
 
-@var{plain} and @var{scram} (``password-based''):
-
-@indentedblock
+@table @asis
+@item @code{plain} and @code{scram} (``password-based'')
 Here, ``password'' refers to your account password, which is usually
 your @samp{NickServ} password.  To make this work, customize
 @code{erc-sasl-user} and @code{erc-sasl-password} or specify the
 @code{:user} and @code{:password} keyword arguments when invoking
 @code{erc-tls}.  Note that @code{:user} cannot be given interactively.
-@end indentedblock
 
-@var{external} (via Client TLS Certificate):
-
-@indentedblock
+@item @code{external} (via Client TLS Certificate)
 This works in conjunction with the @code{:client-certificate} keyword
 offered by @code{erc-tls}.  Just ensure you've registered your
 fingerprint with the network beforehand.  The fingerprint is usually a
@@ -1018,11 +1015,8 @@ of authentication is in effect (being deferred to), so depending on
 the specific application or service, there's a remote chance your
 server has something else in mind.
 @end enumerate
-@end indentedblock
 
-@var{ecdsa-nist256p-challenge}:
-
-@indentedblock
+@item @code{ecdsa-nist256p-challenge}
 This mechanism is quite complicated and currently requires the
 external @samp{openssl} executable, so please use something else if at
 all possible.  Ignoring that, specify your key file (e.g.,
@@ -1036,7 +1030,7 @@ ERC> /msg NickServ set property \
 
 @end example
 (You may be able to omit the @samp{property} subcommand.)
-@end indentedblock
+@end table
 
 @end defopt