libxml2: Clearer wording Homebrew's site-packages

Style fix in PythonInstalled req. that is :recommended.

Formula/libxml2.rb

@@ -19,7 +19,7 @@ class Libxml2 < Formula
     depends_on :libtool
   else
     # 2.9.1 cannot build with Python 2.6: https://github.com/mxcl/homebrew/issues/20249
-    depends_on PythonInstalled.new("2.7", :recommended)
+    depends_on PythonInstalled.new("2.7") => :recommended
   end
 
   fails_with :llvm do
@@ -64,7 +64,7 @@ def caveats
     if build.with? 'python'
       <<-EOS.undent
         Even if this formula is keg_only, the python bindings have been linked
-        into the global site-packages for your convenience.
+        into Homebrew's global site-packages for your convenience.
           #{python.global_site_packages}
 
       EOS