Change style of required packages list

The required packages list should be easier to parse. Updated the style so it's more similar to the ``./configure`` style later in the document.

Signed-off-by: Jarad Olson <brotherdust+github@gmail.com>

doc/developer/building-frr-on-ubuntu1804.rst

@@ -8,9 +8,25 @@ Add packages:
 
 ::
 
-    apt-get install git autoconf automake libtool make gawk libreadline-dev \
+    apt-get install \
-       texinfo pkg-config libpam0g-dev libjson-c-dev bison flex \
+        git \
-       python-pytest libc-ares-dev python3-dev libsystemd-dev python-ipaddr \
+        autoconf \
+        automake \
+        libtool \
+        make \
+        gawk \
+        libreadline-dev \
+        texinfo \
+        pkg-config \
+        libpam0g-dev \
+        libjson-c-dev \
+        bison \
+        flex \
+        python-pytest \
+        libc-ares-dev \
+        python3-dev \
+        libsystemd-dev \
+        python-ipaddr \
         python3-sphinx
 
 Get FRR, compile it and install it (from Git)