Fix invalid reference in CI workflow

Author: djc

.github/workflows/ci.yml

@@ -66,7 +66,7 @@ jobs:
       - name: cargo doc (all features)
         run: cargo doc --all-features --document-private-items
         env:
-          RUSTDOCFLAGS: ${{ matrix.rust_channel == 'nightly' && '-Dwarnings --cfg=docsrs' || '-Dwarnings' }}
+          RUSTDOCFLAGS: ${{ matrix.toolchain == 'nightly' && '-Dwarnings --cfg=docsrs' || '-Dwarnings' }}
 
   check-external-types:
     name: Validate external types appearing in public API