Incorrect example for `[[` command.

[hk]

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS: -g -O2 -Wno-parentheses -Wno-format-security
uname output: Linux hk 4.15.0-62-generic #69-Ubuntu SMP Wed Sep 4 20:55:53
UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
Machine Type: x86_64-pc-linux-gnu

Bash Version: 5.0
Patch Level: 0
Release Status: release

Description:
        On section 3.2.4.2 of Bash Reference Manual, the example on*
[[...]]* (page 13 in the PDF) is incorrect. Specifically, the example say *[[
$line =~ [[:space:]]*?(a)b ]]*  will match values like *'aab'* and*
'aaaaaab*'. But it won't. The operator is* =~*, but the operand on the
right side is a pattern while it should be a regular expression.

Repeat-By:
        [Describe the sequence of events that causes the problem
        to occur.]

Fix:
        [Description of how to fix the problem.  If you don't know a
        fix for the problem, don't include this section.]