correct file open mode for mmap_private man page

mmap/mmap_private.3

@@ -6,7 +6,7 @@ mmap_private \- memory map a file for reading and writing
 
 char* \fBmmap_private\fP(const char* \fIfilename\fR,size_t* \fIfilesize\fR);
 .SH DESCRIPTION
-mmap_private opens \fIfilename\fR for reading and writing, maps the
+mmap_private opens \fIfilename\fR for reading, maps the
 whole file into memory, closes the file, writes the length of the file
 to \fIfilesize\fR and returns a pointer to the mapped file.