[TAP] Fix the documentation of the property `backup_frequency`

				<td>

					<p>Frequency at which the UWS service (that's to say, all its users and jobs) must be backuped.</p>

					<p>Allowed values are: <code>never</code> (no backup will never be done), <code>user_action</code> (each time a user does a writing action, like creating or execution a job), a time (must be positive and not null) in milliseconds.</p>

					<p><em>The value <code>user_action</code> can be used ONLY IF <code>backup_mode=true</code>.</em></p>

					<p><em>The value <code>user_action</code> can be used ONLY IF <code>backup_by_user=true</code>.</em></p>

					<p><em>Default: <code>backup_frequency=never</code> (no backup)</em></p>

				</td>

				<td><ul><li>never <em>(default)</em></li><li>user_action</li><li>3600000 <em>(1 hour)</em></li></ul></td>

#             FULL TAP CONFIGURATION FILE                #

#                                                        #

# TAP Version: 2.1                                       #

# Date: 02 Aug. 2017                                     #

# Date: 24 Jan. 2018                                     #

# Author: Gregory Mantelet (ARI)                         #

#                                                        #

########################################################## 

# Allowed values: never (no backup will never be done ; default), user_action (each time a user does a writing action, like creating or execution a job),

#                 a time (must be positive and not null) in milliseconds.

# 

# The value user_action can be used ONLY IF backup_mode=true.

# The value user_action can be used ONLY IF backup_by_user=true.

# 

# Default: never

backup_frequency = never