replace gtk2 with gtk3 in docs

docs/development/build-instructions-linux.md

@@ -17,7 +17,7 @@ Follow the guidelines below for building Electron on Linux.
 On Ubuntu, install the following libraries:
 
 ```sh
-$ sudo apt-get install build-essential clang libdbus-1-dev libgtk2.0-dev \
+$ sudo apt-get install build-essential clang libdbus-1-dev libgtk-3-dev \
                        libnotify-dev libgnome-keyring-dev libgconf2-dev \
                        libasound2-dev libcap-dev libcups2-dev libxtst-dev \
                        libxss1 libnss3-dev gcc-multilib g++-multilib curl \
@@ -27,7 +27,7 @@ $ sudo apt-get install build-essential clang libdbus-1-dev libgtk2.0-dev \
 On RHEL / CentOS, install the following libraries:
 
 ```sh
-$ sudo yum install clang dbus-devel gtk2-devel libnotify-devel \
+$ sudo yum install clang dbus-devel gtk3-devel libnotify-devel \
                    libgnome-keyring-devel xorg-x11-server-utils libcap-devel \
                    cups-devel libXtst-devel alsa-lib-devel libXrandr-devel \
                    GConf2-devel nss-devel
@@ -36,7 +36,7 @@ $ sudo yum install clang dbus-devel gtk2-devel libnotify-devel \
 On Fedora, install the following libraries:
 
 ```sh
-$ sudo dnf install clang dbus-devel gtk2-devel libnotify-devel \
+$ sudo dnf install clang dbus-devel gtk3-devel libnotify-devel \
                    libgnome-keyring-devel xorg-x11-server-utils libcap-devel \
                    cups-devel libXtst-devel alsa-lib-devel libXrandr-devel \
                    GConf2-devel nss-devel

docs/tutorial/snapcraft.md

@@ -125,7 +125,7 @@ parts:
     source: my-deb.deb
     source-type: deb
     after:
-      - desktop-gtk2
+      - desktop-gtk3
     stage-packages:
       - libasound2
       - libgconf2-4