docs(autoware-interfaces): fix description of base link (#164)

* docs(autoware-interfaces): fix description of base link

Signed-off-by: Takagi, Isamu <isamu.takagi@tier4.jp>

* Update docs/design/autoware-interfaces/components/vehicle-dimensions.md

Signed-off-by: Lalith Vipulananthan <lalith.vipulananthan@tier4.jp>

Co-authored-by: Kenji Miyake <31987104+kenji-miyake@users.noreply.github.com>

Signed-off-by: Takagi, Isamu <isamu.takagi@tier4.jp>
Co-authored-by: Lalith Vipulananthan <lalith.vipulananthan@tier4.jp>
Co-authored-by: Kenji Miyake <31987104+kenji-miyake@users.noreply.github.com>

docs/design/autoware-interfaces/components/vehicle-dimensions.md

@@ -4,7 +4,7 @@
 
 ![Vehicle Axes](images/vehicle_axes.svg){: style="width:500px"}
 
-The `base_link` frame is used very frequently throughout the stack, and is in the center of the rear axle.
+The `base_link` frame is used very frequently throughout the Autoware stack, and is a projection of the rear-axle center onto the ground surface.
 
 - Localization module outputs the `map` to `base_link` transformation.
 - Planning module plans the poses for where the `base_link` frame should be in the future.