Incorrect function prototype in documentation

[Phil Wolff]

In the chapter on Gtk::TreeView, there are two instances of the string "void on_search_equal" that should read "bool on_search_equal".

Ubuntu 17.10/Wayland
libgtkmm-3.0 3.22.2-1

ProblemType: Bug
DistroRelease: Ubuntu 17.10
Package: libgtkmm-3.0-doc 3.22.2-1
ProcVersionSignature: Ubuntu 4.13.0-31.34-generic 4.13.13
Uname: Linux 4.13.0-31-generic x86_64
ApportVersion: 2.20.7-0ubuntu3.7
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Thu Jan 25 06:23:19 2018
InstallationDate: Installed on 2015-07-17 (922 days ago)
InstallationMedia: Ubuntu 15.04 "Vivid Vervet" - Release amd64 (20150422)
PackageArchitecture: all
ProcEnviron:
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: gtkmm3.0
UpgradeStatus: Upgraded to artful on 2017-10-21 (95 days ago)