Improve Travis sample in the docs (#17420)

- printf is better than echo for multiline strings
- ** should be &&
- use line continuation
- Use multiline block

lib/spack/docs/workflows.rst

@@ -1405,11 +1405,12 @@ The main points that are implemented below:
      - export CXXFLAGS="-std=c++11"
 
    install:
-     - if ! which spack >/dev/null; then
+     - |
+       if ! which spack >/dev/null; then
          mkdir -p $SPACK_ROOT &&
          git clone --depth 50 https://github.com/spack/spack.git $SPACK_ROOT &&
-         echo -e "config:""\n  build_jobs:"" 2" > $SPACK_ROOT/etc/spack/config.yaml **
-         echo -e "packages:""\n  all:""\n    target:"" ['x86_64']"
+         printf "config:\n  build_jobs: 2\n" > $SPACK_ROOT/etc/spack/config.yaml &&
+         printf "packages:\n  all:\n    target: ['x86_64']\n" \
                  > $SPACK_ROOT/etc/spack/packages.yaml;
        fi
      - travis_wait spack install cmake@3.7.2~openssl~ncurses