hero: Update CTA

Author: Will Binns

_theme/hero.html

@@ -1,7 +1,7 @@
 <div class="hero">
   <div class="content-container">
     <p class="hero-small-title">Welcome</p>
-    <h1 class="main-title">Understand Bitcoin and start building Bitcoin-based applications.</h1>
+    <h1 class="main-title">Learn Bitcoin and start building Bitcoin-based applications.</h1>
     <div class="hero-nav">
       {{ toctree(maxdepth=1, collapse=False, includehidden=False) }}
     </div>