[mutter] tests/wayland: Update documentation for MUTTER_REF_TEST_UPDATE

[Marge Bot <marge-bot src gnome org>]

commit 27dbf77d198d7ab7fc934b761c0885a6080353c4
Author: Robert Mader <robert mader collabora com>
Date:   Mon May 23 13:55:49 2022 +0200

    tests/wayland: Update documentation for MUTTER_REF_TEST_UPDATE
    
    For some reason `LIBGL_ALWAYS_SOFTWARE=1` does not work as intended
    while `MESA_LOADER_DRIVER_OVERRIDE=swrast` does.
    
    Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1055>

 src/tests/meta-ref-test.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/src/tests/meta-ref-test.c b/src/tests/meta-ref-test.c
index 84ac8876d7..f66b074172 100644
--- a/src/tests/meta-ref-test.c
+++ b/src/tests/meta-ref-test.c
@@ -56,13 +56,13 @@
  * existing reference image is pixel identical to the newly created one.
  *
  * Updating test reference images also requires using a software OpenGL
- * renderer, which can be achieved using LIBGL_ALWAYS_SOFTWARE=1
+ * renderer, which can be achieved using MESA_LOADER_DRIVER_OVERRIDE=swrast
  *
  * For example, for the test case '/path/to/test/case', run the test
  * inside
  *
  * ```
- * env LIBGL_ALWAYS_SOFTWARE=1 MUTTER_REF_TEST_UPDATE='/path/to/test/case`
+ * env MESA_LOADER_DRIVER_OVERRIDE=swrast MUTTER_REF_TEST_UPDATE='/path/to/test/case`
  * ```
  *
  */