To avoid wonky vim highlighting, tweak comment.

git-svn-id: https://tufte-latex.googlecode.com/svn/trunk@149 516e2f36-ce3a-0410-bea4-1d4a03f5df72

tufte-common.def

@@ -1,5 +1,5 @@
 %%
-%% This file contains the code that's common to the Tufte-LaTeX document classes.
+%% This file contains the code that is common to the Tufte-LaTeX document classes.
 %%
 
 \ProvidesFile{tufte-common.def}[2009/12/11 v3.5.0 Common code for the Tufte-LaTeX styles]