README.md: fix build instructions (#1880)

Co-authored-by: dominic <[email protected]>

README.md

@@ -78,7 +78,7 @@ $ cmake -E make_directory "build"
 # Generate build system files with cmake, and download any dependencies.
 $ cmake -E chdir "build" cmake -DBENCHMARK_DOWNLOAD_DEPENDENCIES=on -DCMAKE_BUILD_TYPE=Release ../
 # or, starting with CMake 3.13, use a simpler form:
-# cmake -DCMAKE_BUILD_TYPE=Release -S . -B "build"
+# cmake -DBENCHMARK_DOWNLOAD_DEPENDENCIES=on -DCMAKE_BUILD_TYPE=Release -S . -B "build"
 # Build the library.
 $ cmake --build "build" --config Release
 ```