fix zinc testing instructions

* --binary-dup-excludes is now --binary-dup-exclude-files
* zinc is no longer in BUILD.tools

Testing Done:
have been using these commands to test zinc changes locally

https://travis-ci.org/pantsbuild/pants/builds/112447085

Bugs closed: 2984

Reviewed at https://rbcommons.com/s/twitter/r/3513/

zinc/README.md

@@ -21,9 +21,9 @@ Zinc is built using pants:
 
 To build a jar use:
 
-    ./pants binary.dup --excludes="['rootdoc.txt']" zinc:
+    ./pants binary.dup --exclude-files="['rootdoc.txt']" zinc
 
-To consume a jar, change the zinc target in BUILD.tools. To force pants to re-resolve
+To consume a jar, add a zinc target in BUILD.tools. To force pants to re-resolve
 and re-shade the artifact, use a new `rev` whenever the artifact has changed.
 
     jar_library(name = 'zinc',