fix wording

Author: lucaferranti

app/views/papers/_form.html.erb

@@ -34,7 +34,7 @@
   <div class="row">
     <div class="col">
       <%= f.label "Name of git branch containing the paper, not the path" %>
-      <%= f.text_field :git_branch, placeholder: "Leave blank if paper.md is in the default branch", class: "form-control" %>
+      <%= f.text_field :git_branch, placeholder: "Leave blank if paper.tex is in the default branch", class: "form-control" %>
     </div>
     <div class="col">
       <%= f.label "Software Version" %>