Fix typo in docs workflow and add caching

.github/workflows/docs.yml

@@ -30,11 +30,22 @@ jobs:
         with:
            python-version: "3.10"
 
+      - name: Cache pip
+        uses: actions/cache@v2
+        with:
+          # This path is specific to Ubuntu
+          path: ~/.cache/pip
+          # Look to see if there is a cache hit for the corresponding requirements file
+          key: ${{ runner.os }}-pip-${{ hashFiles('requirements.txt') }}
+          restore-keys: |
+            ${{ runner.os }}-pip-
+            ${{ runner.os }}-
+
       - name: Install dependencies
         run: | 
           python -m pip install --upgrade pip
           pip install -r requirements.txt
-	  pip install -r requirements/docs.txt
+          pip install -r requirements/docs.txt
       - name: Build HTML
         run: | 
           cd docs/