[PATCH v1] ring: correct the comment and figure description

[Haiyue Wang <haiyue.wang@intel.com>]

The index description isn't right, correct it as the Programmer's guide
said.

Also correct the guide's figure description about 'Dequeue First Step'.

Signed-off-by: Haiyue Wang <haiyue.wang@intel.com>
---
 doc/guides/prog_guide/ring_lib.rst | 2 +-
 lib/ring/rte_ring_core.h           | 4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/doc/guides/prog_guide/ring_lib.rst b/doc/guides/prog_guide/ring_lib.rst
index 54e0bb4b68..515a715266 100644
--- a/doc/guides/prog_guide/ring_lib.rst
+++ b/doc/guides/prog_guide/ring_lib.rst
@@ -172,7 +172,7 @@ If there are not enough objects in the ring (this is detected by checking prod_t
 
 .. figure:: img/ring-dequeue1.*
 
-   Dequeue last step
+   Dequeue first step
 
 
 Dequeue Second Step
diff --git a/lib/ring/rte_ring_core.h b/lib/ring/rte_ring_core.h
index 1252ca9546..82b237091b 100644
--- a/lib/ring/rte_ring_core.h
+++ b/lib/ring/rte_ring_core.h
@@ -111,8 +111,8 @@ struct rte_ring_hts_headtail {
  * An RTE ring structure.
  *
  * The producer and the consumer have a head and a tail index. The particularity
- * of these index is that they are not between 0 and size(ring). These indexes
- * are between 0 and 2^32, and we mask their value when we access the ring[]
+ * of these index is that they are not between 0 and size(ring)-1. These indexes
+ * are between 0 and 2^32 -1, and we mask their value when we access the ring[]
  * field. Thanks to this assumption, we can do subtractions between 2 index
  * values in a modulo-32bit base: that's why the overflow of the indexes is not
  * a problem.
-- 
2.35.1