[se-edu#580] AboutUs page: rename profile photo files (se-edu#585)

CS2103 requirements state that profile photos filenames need to be of
the format 'github_id_in_lower_case.png' (e.g. damithc.png).

Let's rename the photos to follow the format, and modify AboutUs.adoc
to point to the new filenames.

docs/AboutUs.adoc

@@ -7,30 +7,30 @@ We are a team based in the http://www.comp.nus.edu.sg[School of Computing, Natio
 == Project Team
 
 === http://www.comp.nus.edu.sg/~damithch[Damith C. Rajapakse]
-image::DamithRajapakse.jpg[width="150", align="left"]
+image::damithc.jpg[width="150", align="left"]
 
 Role: Project Advisor
 
 '''
 
 === http://github.com/lejolly[Joshua Lee]
-image::JoshuaLee.jpg[width="150", align="left"]
+image::lejolly.jpg[width="150", align="left"]
 
 Role: Developer +
 Responsibilities: UI
 
 '''
 
 === http://github.com/yijinl[Leow Yijin]
-image::LeowYijin.jpg[width="150", align="left"]
+image::yijinl.jpg[width="150", align="left"]
 
 Role: Developer +
 Responsibilities: Data
 
 '''
 
 === http://github.com/m133225[Martin Choo]
-image::MartinChoo.jpg[width="150", align="left"]
+image::m133225.jpg[width="150", align="left"]
 
 Role: Developer +
 Responsibilities: Dev Ops
@@ -45,7 +45,7 @@ Responsibilities: Threading
 '''
 
 === http://github.com/yl-coder[You Liang]
-image::YouLiang.jpg[width="150", align="left"]
+image::yl_coder.jpg[width="150", align="left"]
 
 Role: Developer +
 Responsibilities: UI