xmldoc: Avoid whitespace around value for parameter/required.

Otherwise, the value 'false' was not found in the enumerated set of
the XML DTD for the XML attribute 'required' in the XML element
'parameter'. Therefore, DTD validation of the runtime XML failed.

ASTERISK-29790

Change-Id: Id13f230ad65a70dd8c2e3ae9ac85d1e841aed03e

doc/appdocsxml.xslt

@@ -162,9 +162,7 @@
             <para>The video source mode for the bridge.</para>
         </xsl:element>
         <xsl:element name="parameter">
-            <xsl:attribute name="required">
+            <xsl:attribute name="required">false</xsl:attribute>
-                false
-            </xsl:attribute>
             <xsl:attribute name="name">
                 <xsl:value-of select="concat(@prefix, 'BridgeVideoSource')" />
             </xsl:attribute>