Fix a typo reported by Florian Ernst.

man/zip_source_file.mdoc

@@ -86,7 +86,7 @@ Reading from the source returns as much data as is there at that time
 can do various optimizations if the size of a source is known when
 it's created, so
 .Dv ZIP_LENGTH_TO_END
-is preferrable.
+is preferable.
 If you deal with files that are likely to change while you are
 processing them, you can use the less efficient
 .Dv ZIP_LENGTH_UNCHECKED .