Update algo_walkthrough.rst (cartographer-project#1545)

Fix typos.

Signed-off-by: zhenzhenxiang <[email protected]>

docs/source/algo_walkthrough.rst

@@ -276,9 +276,9 @@ The resulting net is called the "*pose graph*".
 
     Constraints can be visualized in RViz, it is very handy to tune global SLAM. One can also toggle ``POSE_GRAPH.constraint_builder.log_matches`` to get regular reports of the constraints builder formatted as histograms.
 
-- Non-global constraints (also known as inter submaps constraints) are built automatically between nodes that are closely following each other on a trajectory.
+- Non-global constraints (also known as intra submaps constraints) are built automatically between nodes that are closely following each other on a trajectory.
   Intuitively, those "*non-global ropes*" keep the local structure of the trajectory coherent.
-- Global constraints (also referred to as loop closure constraints or intra submaps contraints) are regularly searched between a new submap and previous nodes that are considered "*close enough*" in space (part of a certain **search window**) and a strong fit (a good match when running scan matching).
+- Global constraints (also referred to as loop closure constraints or inter submaps contraints) are regularly searched between a new submap and previous nodes that are considered "*close enough*" in space (part of a certain **search window**) and a strong fit (a good match when running scan matching).
   Intuitively, those "*global ropes*" introduce knots in the structure and firmly bring two strands closer.
 
 .. code-block:: lua