Update typo in pre-release workflow

Signed-off-by: Brian Warner <[email protected]>

.github/workflows/pre-release.yml

@@ -51,7 +51,7 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        python -m pip install -r sphinx sphinx_rtd_theme myst_parser
+        python -m pip install sphinx sphinx_rtd_theme myst_parser
     - name: Generate documentation
       run: |
         sphinx-build -b html docsrc docs -E -d "docsrc/_doctrees"