Fix typo in CI (#358)

.env

@@ -1,2 +1,2 @@
 MDBOOK_VERSION=0.4.28
-MDBOOK_I8N_HELPERS_VERSION=0.1.0
+MDBOOK_I18N_HELPERS_VERSION=0.1.0

.github/workflows/install-mdbook/action.yml

@@ -12,11 +12,11 @@ runs:
         echo "::set-output name=MDBOOK_VERSION::${MDBOOK_VERSION}"
       shell: bash
 
-    - name: Read mdbook-i8n-helpers version from .env
-      id: mdbook-i8n-helpers-version
+    - name: Read mdbook-i18n-helpers version from .env
+      id: mdbook-i18n-helpers-version
       run: |
         . ./.env
-        echo "::set-output name=MDBOOK_I8N_HELPERS_VERSION::${MDBOOK_I8N_HELPERS_VERSION}"
+        echo "::set-output name=MDBOOK_I18N_HELPERS_VERSION::${MDBOOK_I18N_HELPERS_VERSION}"
       shell: bash
 
     # The --locked flag is important for reproducible builds.
@@ -25,5 +25,5 @@ runs:
       shell: bash
 
     - name: Install i18n-helpers
-      run: cargo install mdbook-i18n-helpers --locked --version '${{ steps.mdbook-i8n-helpers-version.outputs.MDBOOK_I8N_HELPERS_VERSION }}'
+      run: cargo install mdbook-i18n-helpers --locked --version '${{ steps.mdbook-i18n-helpers-version.outputs.MDBOOK_I18N_HELPERS_VERSION }}'
       shell: bash