Corrected README markdown

Fixed some invalid mixing of Markdown and HTML syntax in the README.md file.
Reformatted instruction mix output

Made the output of the instruction-mix histogram look a bit nicer.
Fixed memory-freeing bug

Due to a shallow copy in the code, the inst_mix_histo *pointer* was
getting copied from one object to another, then double-freed.  This
version allocates inst_mix_histo as a fixed-size array both to solve
that problem and to reduce the number of memory allocations (at the
expense of storage requirements when -bf-inst-mix isn't used).
Replaced bf_total_inst_count with NUM_OPCODES

Now that the NUM_OPCODES compiler constant exists, there is no longer
a need for a computed bf_total_inst_count variable so this was
removed.
Fixed instruction-mix histogram reset bug

The instruction-mix histogram wasn't being cleared along with all of the other counters.  The instrumentation code now clears it unconditionally (i.e., without checking must_clear) whenenver -bf-inst-mix is specified.
Updated sample output in README.md

Attached sample program output that includes the output of -bf-inst-mix.