fix frozen string error

Author: C24-AK

lib/puppet/provider/mysql_user/mysql.rb

@@ -172,10 +172,10 @@ def password_hash=(string)
       %r{^\*|^$}.match?(string) || %r{0x[A-F0-9]+$}.match?(string)
 
       sql = "ALTER USER #{merged_name} IDENTIFIED WITH"
-      sql << if plugin == 'caching_sha2_password'
-               " 'caching_sha2_password' AS X'#{string[2..-1]}'"
+      sql += if plugin == 'caching_sha2_password'
+               " caching_sha2_password AS X'#{string[2..-1]}'"
              else
-               " 'mysql_native_password' AS '#{string}'"
+               " mysql_native_password AS '#{string}'"
              end
       self.class.mysql_caller(sql, 'system')
     else