Fixed the gzip synopsis.

Doc/lib/libgzip.tex

 \section{Standard Module \module{gzip}}
 \declaremodule{standard}{gzip}
 
-\modulesynopsis{None}
+\modulesynopsis{Interfaces for \program{gzip} compression and
+decompression using file objects.}
 
 
 The data compression provided by the \code{zlib} module is compatible