Fix url to API doc in readme

[jrutgeer]

Changed README to point to the API doc via the rclcpp package info page at the ROS Index.

[fujitatomoya]

@jrutgeer thanks for the contribution, this PR surely fixes one link, which is good. but i can see there are many places use docs.ros2.org, so those probably need to be changed as well. besides we are having discussion on ros2/ros2#1354.

[clalancette]

Looks good to me, but I'll wait a bit longer before merging to get other opinions.

[clalancette]

We've had no further reviews, so I'm happy to merge it as-is.

@jrutgeer I just realized that the DCO bot is unhappy because the commit does not have the Signed-off-by: line. Can you please rebase, add that, and then force-push it? Once that is done we can merge.

[jrutgeer]

@clalancette I tried to do that, but I think I failed.
Also when I press compare above it seems that 18 files were changed??
I don´t know how come I could have had write access to this repository and not only my local fork?

[clalancette]

I don´t know how come I could have had write access to this repository and not only my local fork?

You only have access to your local fork. Because you did a merge, you brought in changes that happened since your fork.

In this case, the easiest thing to do is to just make a brand-new commit with a Signed-off-by line, and then open a new PR.