Fix another typo in a comment

Noted by Thom Brown

src/backend/storage/file/fd.c

@@ -47,7 +47,7 @@
  * and there is a fixed limit on the number of files that can be opened using
  * these functions at any one time.
  *
- * Finally, BasicOpenFile is a just thin wrapper around open() that can
+ * Finally, BasicOpenFile is just a thin wrapper around open() that can
  * release file descriptors in use by the virtual file descriptors if
  * necessary. There is no automatic cleanup of file descriptors returned by
  * BasicOpenFile, it is solely the caller's responsibility to close the file