Docs: datarep argument to MPI_Pack_external_size is const

Signed-off-by: Joseph Schuchart <schuchart@icl.utk.edu>

Author: devreal

docs/man-openmpi/man3/MPI_Pack_external_size.3.rst

@@ -21,7 +21,7 @@ C Syntax
 
    #include <mpi.h>
 
-   int MPI_Pack_external_size(char *datarep, int incount,
+   int MPI_Pack_external_size(const char *datarep, int incount,
    	MPI_Datatype datatype, MPI_Aint *size)