BugFix: user name contains invalid characters

There is an typo in the policy "[a-zA-z_]" configuration.
It it should be "[a-zA-Z_]"

Tested: Create a username contains invalid characters like: ^, [, or ].

Before: invalid character can be added as username
ipmitool user list 1
ID  Name      Callin
4   ^test     true
5   [test     true
7   ]test     true
8   _test     true

After: error will be returned once user name has invalid character.
ipmitool user set name 4 ^test4
Set User Name command failed (user 4, name ^test4): Unspecified error

Change-Id: I28a7f36f10b44b59909536214e6688c1895ac14f
Signed-off-by: nichanghao.nch <nichanghao@linux.alibaba.com>
diff --git a/user_channel/user_mgmt.cpp b/user_channel/user_mgmt.cpp
index 4ec61d5..97e6a8c 100644
--- a/user_channel/user_mgmt.cpp
+++ b/user_channel/user_mgmt.cpp
@@ -565,7 +565,7 @@
         return false;
     }
     if (!std::regex_match(userName.c_str(),
-                          std::regex("[a-zA-z_][a-zA-Z_0-9]*")))
+                          std::regex("[a-zA-Z_][a-zA-Z_0-9]*")))
     {
         log<level::ERR>("Unsupported characters in user name");
         return false;