Fix file encoding of bigint-func.cc

Switch the file encoding of bigint-func.cc from ISO-8859 text to UTF-8
Unicode text.  Inspection using `file` suggests this is the only source
file with wrong encoding.

Fixes: #7509

Author: tautschnig

src/big-int/bigint-func.cc

@@ -2,7 +2,7 @@
 // Author: Dirk Zoller
 // References:
 //
-// [1] Hüttenhofer/Lesch/Peyerimhoff, Mathematik in Anwendung mit C++
+// [1] Hüttenhofer/Lesch/Peyerimhoff, Mathematik in Anwendung mit C++
 //     Heidelberg, Wiesbaden 1994
 
 #include "bigint.hh"