_content: fix heading level hierarchy

Change-Id: Icb333a465491af0c70bdfc5c1134a2c707711f12
Reviewed-on: https://go-review.googlesource.com/c/website/+/506997
Reviewed-by: Neelesh Bodas <neelesh@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Prudhvi Krishna Chintala <prudhvikrishna@google.com>
Reviewed-by: Nooras Saba‎ <saba@golang.org>
Run-TryBot: Jamal Carvalho <jamal@golang.org>

_content/css/styles.css

@@ -1241,7 +1241,7 @@ a.GettingStartedGo-resourceItemTitle {
   font-size: 0.8125rem;
 }
 .WhoUses-header h4,
-.WhyGo-headerH4,
+.WhyGo-subheader,
 .LearnGo-header h4,
 .WhoUses-subheader {
   color: var(--color-text);
@@ -1253,7 +1253,7 @@ a.GettingStartedGo-resourceItemTitle {
 }
 @media only screen and (min-width: 38rem) {
   .WhoUses-header h4,
-  .WhyGo-headerH4,
+  .WhyGo-subheader,
   .LearnGo-header h4,
   .WhoUses-subheader {
     font-size: 1rem;

_content/index.md

@@ -186,9 +186,9 @@ func main() {
   <div class="WhyGo-gridContainer">
     <div class="WhyGo-header">
       <h2 class="WhyGo-headerH2">What’s possible with Go</h2>
-      <h4 class="WhyGo-headerH4">
+      <p class="WhyGo-subheader">
         Use Go for a variety of software development purposes
-      </h4>
+      </p>
     </div>
     <ul class="WhyGo-reasons">
       {{- range first 4 (data "/resources.yaml")}}