Fix copy and paste error.

Change-Id: Ie28196d44e163d26b09131687ef56f5f8cc77ac7

internal/ceres/file.cc

diff --git a/internal/ceres/file.cc b/internal/ceres/file.cc
index 5fc9d22..14ddf8e 100644
--- a/internal/ceres/file.cc
+++ b/internal/ceres/file.cc
@@ -48,7 +48,7 @@
 }
 
 void ReadFileToStringOrDie(const string &filename, string *data) {
-  FILE* file_descriptor = file_descriptor = fopen(filename.c_str(), "r");
+  FILE* file_descriptor = fopen(filename.c_str(), "r");
 
   if (!file_descriptor) {
     LOG(FATAL) << "Couldn't read file: " << filename;