docs/about/deprecated.rst: spelling fix: becase

Fixes: 864128df "migration: Deprecate old compression method"
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

docs/about/deprecated.rst

@@ -514,5 +514,5 @@ old compression method (since 8.2)
 
 Compression method fails too much.  Too many races.  We are going to
 remove it if nobody fixes it.  For starters, migration-test
-compression tests are disabled becase they fail randomly.  If you need
+compression tests are disabled because they fail randomly.  If you need
 compression, use multifd compression methods.