list awk in the dependencies needed for installation

(it is installed by apt-get install gawk, I believe)

Fixes #557.

R=rsc
CC=golang-dev
https://golang.org/cl/194050

doc/install.html

@@ -189,13 +189,13 @@ $ hg clone -r release https://go.googlecode.com/hg/ $GOROOT
 
 <p>The Go tool chain is written in C.  To build it, you need
 to have GCC, the standard C libraries, the parser generator Bison,
-<tt>make</tt> and the text editor <tt>ed</tt> installed.  On OS X, they can be
+<tt>make</tt>, <tt>awk</tt>, and the text editor <tt>ed</tt> installed.  On OS X, they can be
 installed as part of
 <a href="http://developer.apple.com/TOOLS/Xcode/">Xcode</a>.  On Linux,
 </p>
 
 <pre>
-$ sudo apt-get install bison gcc libc6-dev ed make
+$ sudo apt-get install bison gcc libc6-dev ed gawk make
 </pre>
 
 <p>