Replace string format char

[tlamb96]

Fixes the following error when dump is called on a ROP object:

Traceback (most recent call last):
  File "myrop.py", line 17, in <module>
    print rop.dump()
  File "/usr/local/lib/python2.7/dist-packages/pwnlib/rop/rop.py", line 831, in dump
    return self.build().dump()
  File "/usr/local/lib/python2.7/dist-packages/pwnlib/rop/rop.py", line 705, in build
    stack.describe(self.describe(slot))
  File "/usr/local/lib/python2.7/dist-packages/pwnlib/rop/rop.py", line 632, in describe
    return str(object)
  File "/usr/local/lib/python2.7/dist-packages/pwnlib/rop/call.py", line 238, in __str__
    name = self.name or fmt & self.target
TypeError: unsupported operand type(s) for &: 'str' and 'int'

[zachriggle]

Can you also put parens around the (foo % bar) expression?

[tlamb96]

@zachriggle updated.