DOC: Add the correct punctuation to i.e.

Add missing period to ie.: i.e.

Author: Jon Haitz Legarreta

SoftwareGuide/Latex/DevelopmentGuidelines/WriteAFilter.tex

@@ -146,7 +146,7 @@ \section{Overview of Filter Creation}
 
   \item \code{LightObject}: abstract base for filters that don't fit well
   anywhere else in the class hierarchy.  Also useful for ``calculator''
-  filters; ie. a sink filter that takes an input and calculates a result
+  filters; i.e. a sink filter that takes an input and calculates a result
   which is retrieved using a \code{Get()} method.
 
 \end{itemize}