.gitattributes: Fix LFS paths in docs

There was a typo in blobs paths in docs. These paths needs to match
blobs recursively in the docs directory.

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>

.gitattributes

-docs/*.png filter=lfs diff=lfs merge=lfs -text
-docs/*.jpg filter=lfs diff=lfs merge=lfs -text
+docs/**/*.png filter=lfs diff=lfs merge=lfs -text
+docs/**/*.jpg filter=lfs diff=lfs merge=lfs -text