Fix naming of headshot

Author: brikis98

_data/testimonials.yml

@@ -1,6 +1,6 @@
 - name: "TSet"
   logo: "logos-clients/logo-tset.png"
-  headshot: "headshot-markus-berger.jpeg"
+  headshot: "headshot-markus-berger.jpg"
   content: The quality that Gruntwork produces and maintains is outstanding. It has proven time and time again to be a huge accelerator bringing companies forward in terms of stability and quality. It also enables closer collaboration across the engineering organisation by providing easy to use battle tested modules in a "plug-and-play" manner, enabling us to take care of the core business, generating more value across the board, and not being slowed down by unnecessary "grunt work."
   author: Markus Burger, DevOps Team Lead