fix(gnokii): use double \\ in [...] for POSIX.1-2024 recommendation

Currently, in POSIX-bracket expressions [...] in the POSIX sed regular
expressions, a backslash is treated literally, i.e., a single
backslash is enough to include it in the set of characters.  However,
according to a newly added paragraph "APPLICATION USAGE/p5 - sed -
POSIX Issue 8 XCU", POSIX may allow implementations to interpret
backslashes in bracket expressions bracket in the future (in a similar
way as POSIX awk processes).  It recommends using two backslashes in
bracket expressions for the future compatibility.

https://pubs.opengroup.org/onlinepubs/9799919799/utilities/sed.html#tag_20_109_16

Author: akinomyoga

completions-core/gnokii.bash

@@ -219,9 +219,11 @@ _comp_cmd_gnokii()
     _comp_compgen -Rv all_cmd help -- --help all
 
     # these 2 below are allowed in combination with others
+    # Note: POSIX.1-2024 started to recommend using two backslashes \\ in
+    # bracket expressions [...] in POSIX sed for the future compatibility.
     local main_cmd
     _comp_split -l main_cmd "$(printf '%s\n' "${all_cmd[@]}" |
-        command sed -e '/--config/d;/--phone/d;s/[][\(){}|^$*+?.]/\\&/g')"
+        command sed -e '/--config/d;/--phone/d;s/[][\\(){}|^$*+?.]/\\&/g')"
     # don't provide main command completions if one is
     # already on the command line
     local IFS='|'