Fix some commas (#136)

There was one missed Oxford comma in a heading

Another heading had an extra comma

And there was a run-on sentence that should have been two

There were a bunch more run-on sentences; looking at all the "and"s in
the document those really stand out, but I don't know how to fix those
without digging deeper and that's not appropriate at this stage of the
process.

index.html

@@ -219,7 +219,7 @@ <h3 id="allpeople" data-export="" data-dfn-type="dfn">
 
 <section>
   <h3 id="privacy" data-export="" data-dfn-type="dfn">
-The web is secure, and respects people's privacy
+The web is secure and respects people's privacy
   </h3>
   <p>
 When we add features to the web platform,
@@ -293,8 +293,8 @@ <h3 id="sustainable" data-export="" data-dfn-type="dfn">
   </h3>
   <p>
 Web technologies may have overall positive environmental impacts
-as well as negative impacts,
-and these can change over time and vary geographically as
+as well as negative impacts.
+These can change over time and vary geographically as
 both web and environmental technologies develop.
 We will endeavor not to do further harm to the environment
 when we introduce new technologies to the web and keep in mind
@@ -324,7 +324,7 @@ <h3 id="transparent" data-export="" data-dfn-type="dfn">
 
 <section>
   <h3 id="multi" data-export="" data-dfn-type="dfn">
-The web is multi-browser, multi-OS and multi-device
+The web is multi-browser, multi-OS, and multi-device
   </h3>
   <p>
 We will not create web technologies that encourage the creation of websites