Change README.md so that code works in ZSH.
Change README.md so that code works in ZSH.

ZSH has an supports suffixes in variable substitution.

:r returns the root of a filename

example:
A=ab.c
echo $A:r
> ab

Change-Id: I89fa7bf0401ea7e05f2d1fbc97bf6533dd8584b8

GoogleCloudPlatform/opentelemetry-operations-e2e-testing