docs(readme): fix output directory
docs(readme): fix output directory

Fix the output directory relative so it can run anywhere without errors. Before it was giving this error:
```js
/Users/admejiar/Code/meanshop2/server/node_modules/hummus-recipe/lib/Recipe.js:65
            this.writer = hummus.createWriter(this.output, {
                                 ^

TypeError: Unable to create PDF file, make sure that output file target is available
    at Recipe._createWriter (/Users/admejiar/Code/meanshop2/server/node_modules/hummus-recipe/lib/Recipe.js:65:34)
    at new Recipe (/Users/admejiar/Code/meanshop2/server/node_modules/hummus-recipe/lib/Recipe.js:59:14)
    at Object.<anonymous> (/Users/admejiar/Code/meanshop2/server/index.js:2:16)
    at Module._compile (internal/modules/cjs/loader.js:701:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:712:10)
    at Module.load (internal/modules/cjs/loader.js:600:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:539:12)
    at Function.Module._load (internal/modules/cjs/loader.js:531:3)
    at Function.Module.runMain (internal/modules/cjs/loader.js:754:12)
    at startup (internal/bootstrap/node.js:283:19)
```
chunyenHuang/hummusRecipe,chunyenHuang/hummusRecipe