Vulkan:Clarify volk license file name

Need to explicitly specifly volk LICENSE.md filename in README.chromium
file.

Bug: angleproject:4225
Change-Id: I5d71ef0c28063f6e036f94ddb7d2c9e289fe79d2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1993397
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Tobin Ehlis <tobine@google.com>

src/third_party/volk/README.chromium

@@ -3,6 +3,7 @@ Short Name: volk
 URL: https://github.com/zeux/volk
 Version: 0
 License: MIT
+License File: LICENSE.md
 Security Critical: yes
 
 Description: