Update build.yml

Author: xemlock

.github/workflows/build.yml

@@ -43,7 +43,7 @@ jobs:
             - name: Install dependencies
               run: |
                 echo PHP_VERSION: ${{ env.PHP_VERSION }}
-                composer require --no-update ezyang/htmlpurifier:<=4.17.0
+                composer require --no-update ezyang/htmlpurifier:\<=4.17.0
                 composer install
 
             - name: Run tests