readme: prettify text on the GitHub Action badges

The displayed names now corresponds to the `name` field in the workflow
files:

* `fast_testing` -> `Linux testing`
* `osx_testing` -> `OS X testing`
* `packaging` -> `Packaging`

See [1] for the reference.

[1]: https://docs.github.com/en/actions/monitoring-and-troubleshooting-workflows/adding-a-workflow-status-badge

Follows up PR #163

Author: Totktonada

README.rst

@@ -2,15 +2,15 @@
                             Tarantool C client libraries
 -------------------------------------------------------------------------------
 
-.. image:: https://github.com/tarantool/tarantool-c/workflows/fast_testing/badge.svg
+.. image:: https://github.com/tarantool/tarantool-c/actions/workflows/fast_testing.yml/badge.svg
     :target: https://github.com/tarantool/tarantool-c/actions/workflows/fast_testing.yml?query=branch%3Amaster
     :alt: Linux testing
 
-.. image:: https://github.com/tarantool/tarantool-c/workflows/osx_testing/badge.svg
+.. image:: https://github.com/tarantool/tarantool-c/actions/workflows/osx_testing.yml/badge.svg
     :target: https://github.com/tarantool/tarantool-c/actions/workflows/osx_testing.yml?query=branch%3Amaster
     :alt: OS X testing
 
-.. image:: https://github.com/tarantool/tarantool-c/workflows/packaging/badge.svg
+.. image:: https://github.com/tarantool/tarantool-c/actions/workflows/packaging.yml/badge.svg
     :target: https://github.com/tarantool/tarantool-c/actions/workflows/packaging.yml?query=branch%3Amaster
     :alt: Packaging