Comment nit

[Sean Bruno]

I noted a small nit in the comments of sys/dev/e1000/if_igb.h

Index: if_igb.h
===================================================================
--- if_igb.h	(revision 234466)
+++ if_igb.h	(working copy)
@@ -52,7 +52,7 @@
 #define IGB_MAX_TXD		4096
 
 /*
- * IGB_RXD: Maximum number of Transmit Descriptors
+ * IGB_RXD: Maximum number of Receive Descriptors
  *
  *   This value is the number of receive descriptors allocated by the driver.
  *   Increasing this value allows the driver to buffer more incoming packets.