Fix typo

Author: raphjaph

ci/README.md

@@ -178,7 +178,7 @@ jobs:
 Have a file in the repository root with the nightly toolchain version to use.
 
 ```bash
-$ cat nightly_version
+$ cat nightly-version
 nightly-2024-04-30
 ```
 
@@ -195,7 +195,7 @@ jobs:
         uses: actions/checkout@v4
       - name: Read nightly version
         id: read_toolchain
-        run: echo "nightly_version=$(cat nightly_version)" >> $GITHUB_OUTPUT
+        run: echo "nightly_version=$(cat nightly-version)" >> $GITHUB_OUTPUT
 
   Nightly:                      # 2 jobs, one per manifest.
     name: Test - nightly toolchain