Fixing documentation.

README.md

@@ -494,7 +494,7 @@ ros2 run rqt_image_view rqt_image_view
 ros2 run lrs_exec tst_command --ros-args -r __ns:=/ex0 -p command:=test-seq-1
 ```
 
-This will construct a tree im the TST factory and start execution of it. The tree as JSON is:
+This will construct a tree in the TST factory and start execution of it. The tree as JSON is:
 ```
 {
     "children": [