tools/gn: Update docs to refer to 'public_configs'.

tools/gn/docs/quick_start.md

 # GN Quick Start guide
 
 [TOC]
 
 ## Running GN
 
 You just run `gn` from the command line. There is a script in
 depot\_tools (which is presumably on your path) with this name. The
 script will find the binary in the source tree containing the current
 directory and run it.
@@ skipping 230 matching lines @@
   ]
   all_dependent_configs = [
     ":hello_config"
   ]
 }
 ```
 
 This applies the `hello_config` to the `hello` target itself, plus all
 targets that depend on transitively depend on the current one. Now
 everybody that depends on us will get our settings. You can also set
-`direct_dependent_configs` which applies only to targets that directly
+`public_configs` which applies only to targets that directly
 depend on your target (not transitively).
 
 Now if you compile and run, you'll see the new version with two people:
 
 ```
 > ninja -C out/Default say_hello
 ninja: Entering directory 'out/Default'
 [1/1] Regenerating ninja files
 [4/4] LINK say_hello
 > out/Default/say_hello
@@ skipping 58 matching lines @@
 line flag. This will output a summary of timings for various things.
 
 You can also make a trace of how the build files were executed:
 
 ```
 gn --tracelog=mylog.trace
 ```
 
 and you can load the resulting file in Chrome's `about:tracing` page to
 look at everything.