docs: Update contributor count to over 500.

README.md

@@ -5,7 +5,7 @@ immediacy of real-time chat with the productivity benefits of threaded
 conversations. Zulip is used by open source projects, Fortune 500 companies,
 large standards bodies, and others who need a real-time chat system that
 allows users to easily process hundreds or thousands of messages a day. With
-over 300 contributors merging over 500 commits a month, Zulip is also the
+over 500 contributors merging over 500 commits a month, Zulip is also the
 largest and fastest growing open source group chat project.
 
 [![CircleCI branch](https://img.shields.io/circleci/project/github/zulip/zulip/master.svg)](https://circleci.com/gh/zulip/zulip)

docs/overview/gsoc-ideas.md

@@ -23,7 +23,7 @@ source.
 
 Zulip has gained a considerable amount of traction since it was
 [released as open source software][oss-release] in late 2015, with
-code contributions from [over 450 people](https://zulipchat.com/team)
+code contributions from [over 500 people](https://zulipchat.com/team)
 from all around the world.  Thousands of people use Zulip every single
 day, and your work on Zulip will have impact on the daily experiences
 of a huge and rapidly growing number of people.

templates/zerver/team.html

@@ -30,7 +30,7 @@
             <div class="team">
                 <h1>The core team at Kandra Labs</h1>
                 <p>
-                    Over 400 people have contributed to the Zulip
+                    Over 500 people have contributed to the Zulip
                     codebase, from high school students to 30 year
                     industry veterans, from people launching new careers
                     to people looking for community. Meet the leadership