doc: fix typo in gzip.go (nytimes#92)

* doc: fix typo in gzip.go

* using the proper tense

thanks for finding this!

gzip.go

@@ -81,7 +81,7 @@ type GzipResponseWriter struct {
 
 	code int // Saves the WriteHeader value.
 
-	minSize int    // Specifed the minimum response size to gzip. If the response length is bigger than this value, it is compressed.
+	minSize int    // Specifies the minimum response size to gzip. If the response length is bigger than this value, it is compressed.
 	buf     []byte // Holds the first part of the write before reaching the minSize or the end of the write.
 	ignore  bool   // If true, then we immediately passthru writes to the underlying ResponseWriter.