replaced "Expressive" with "expressive" for a cleaner look

R=r
http://go/go-review/1024010

doc/root.html

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-      <div><span style="font-size: 2em; font-weight: bold;">a systems programming language</span><br><span style="font-size: 1.5em;">Expressive, concurrent, garbage-collected</span></div>
+      <div><span style="font-size: 2em; font-weight: bold;">a systems programming language</span><br><span style="font-size: 1.5em;">expressive, concurrent, garbage-collected</span></div>
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