Fix typo in xlogreader.h and procarray.c

Hi

I found some possible typo in procarray.c and xlogreader.h

- * For VACUUM separate horizons (used to to decide which deleted tuples must
+ * For VACUUM separate horizons (used to decide which deleted tuples must

- *        Callers supply a page_read callback if they want to to call
+ *        Callers supply a page_read callback if they want to call

Best regards,
houzj