README: fix links

The Developer Guide link was pointing to the Gesture Properties page, so
fixed that, and replaced "master" with "HEAD" in the others for
future-proofing and to avoid oppressive language.

BUG=b:162024466
TEST=check the links go where they say they do

Change-Id: Ifb60835f9ce760940f499336a12514b8cccde9e5
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/touchpad-tests/+/2316133
Tested-by: Harry Cutts <hcutts@chromium.org>
Auto-Submit: Harry Cutts <hcutts@chromium.org>
Reviewed-by: Sean O'Brien <seobrien@chromium.org>
Commit-Queue: Sean O'Brien <seobrien@chromium.org>
diff --git a/README.md b/README.md
index b0bc3a8..69f279e 100644
--- a/README.md
+++ b/README.md
@@ -9,8 +9,8 @@
 containing [gesture properties] to set while the Gestures library runs, and a
 Python function which verifies the output and returns a test score.
 
-[gestures-lib]: https://chromium.googlesource.com/chromiumos/platform/gestures/+/master/
-[gesture properties]: https://chromium.googlesource.com/chromiumos/platform/gestures/+/master/docs/gesture_properties.md
+[gestures-lib]: https://chromium.googlesource.com/chromiumos/platform/gestures/+/HEAD/
+[gesture properties]: https://chromium.googlesource.com/chromiumos/platform/gestures/+/HEAD/docs/gesture_properties.md
 
 ## Setting up
 
@@ -23,7 +23,7 @@
 $ sudo make setup-in-place
 ```
 
-[developer guide]: https://chromium.googlesource.com/chromiumos/platform/gestures/+/master/docs/gesture_properties.md
+[developer guide]: https://chromium.googlesource.com/chromiumos/docs/+/HEAD/developer_guide.md
 
 ## Running tests