fix #1790; npm instead of yarn

Author: mbostock

docs/getting-started.md

@@ -112,7 +112,7 @@ This command will ask you a series of questions in order to initialize your new
 <span class="muted">│</span>  <span class="muted">Yes, include sample files</span>
 <span class="muted">│</span>
 <span class="green">◇</span>  Install dependencies?
-<span class="muted">│</span>  <span class="muted">Yes, via yarn</span>
+<span class="muted">│</span>  <span class="muted">Yes, via npm</span>
 <span class="muted">│</span>
 <span class="green">◇</span>  Initialize a git repository?
 <span class="muted">│</span>  <span class="muted">Yes</span>
@@ -122,7 +122,7 @@ This command will ask you a series of questions in order to initialize your new
 <span class="green">◇</span>  Next steps… <span class="muted">──────────╮</span>
 <span class="muted">│</span>                        <span class="muted">│</span>
 <span class="muted">│</span>  <span class="focus">cd hello-framework</span>    <span class="muted">│</span>
-<span class="muted">│</span>  <span class="focus">yarn dev</span>              <span class="muted">│</span>
+<span class="muted">│</span>  <span class="focus">npm run dev</span>           <span class="muted">│</span>
 <span class="muted">│</span>                        <span class="muted">│</span>
 <span class="muted">├────────────────────────╯</span>
 <span class="muted">│</span>