ci: fix typo in arguments of cmake

(aargh, sorry)

.github/workflows/vmactions.yml

@@ -45,6 +45,6 @@ jobs:
 
         run: |
           cmake -S . -B build -GNinja \
-            -SDL_CHECK_REQUIRED_INCLUDES="/usr/local/include" \
+            -DSDL_CHECK_REQUIRED_INCLUDES="/usr/local/include" \
             -DSDL_CHECK_REQUIRED_LINK_OPTIONS="-L/usr/local/lib"
           cmake --build build --verbose -- -j`sysctl -n hw.ncpu`