fix font name

Author: marcoow

.github/workflows/ci.yml

@@ -32,10 +32,10 @@ jobs:
       - name: Use Fallback font for fork PRs
         if: "github.event.pull_request.head.repo.fork"
         run: |
-          sed -i 's/CoreSansA45.ttf/Open Sans/g' book/book.toml
-          sed -i 's/CoreSansA45It.ttf/Open Sans/g' book/book.toml
-          sed -i 's/CoreSansA65.ttf/Open Sans/g' book/book.toml
-          sed -i 's/CoreSansA65It.ttf/Open Sans/g' book/book.toml
+          sed -i 's/CoreSansA45.ttf/OpenSans/g' book/book.toml
+          sed -i 's/CoreSansA45It.ttf/OpenSans/g' book/book.toml
+          sed -i 's/CoreSansA65.ttf/OpenSans/g' book/book.toml
+          sed -i 's/CoreSansA65It.ttf/OpenSans/g' book/book.toml
           cat book/book.toml
       - uses: actions-rust-lang/setup-rust-toolchain@v1
       - name: Install exercise plugin