Change a couple confusing terms

www/big-picture/mission.spt

@@ -6,7 +6,7 @@ nav_children = []
 Our mission is to enable an economy of gratitude, generosity, and love.
 
 
-## Exposition
+## Explanation
 
 *by Chad Whitacre*
 

www/big-picture/welcome.spt

@@ -8,7 +8,7 @@ for us means that we want to invite and empower *everyone* to participate in
 running our company. We think of people in relation to Gittip in several roles:
 
   - **users**---people with an account on Gittip
-  - **confabulators**(!)---people conversing in [our space](/appendices/channels)
+  - **contributors**---people conversing in [our space](/appendices/channels)
   - **owners**---users who buy into [our mission](./mission)
   - **staff**---owners who are on [our payroll](https://www.gittip.com/Gittip/members/)
 
@@ -21,7 +21,7 @@ To be a **user**, [create an account](https://www.gittip.com/)! We'll expect
 you to abide by our [terms and
         conditions](https://www.gittip.com/about/terms/).
 
-To be a **confabulator**, simply [enter our space](/appendices/channels) and
+To be a **contributor**, simply [enter our space](/appendices/channels) and
 join the conversation! We'll expect you to [behave well](/howto/behave-well)
 and to be willing to [resolve conflict](/howto/resolve-conflict) when it
 arises.
@@ -38,4 +38,4 @@ to be comfortable with our [team take
 
 Thanks for your interest in helping out with Gittip! Hopefully this gets you
 pointed in the right direction to get involved. The next step is to read the
-links on this page and then confabulate! :^)
+links on this page and then contribute! :^)