Correct method name for setting DB engine

[Torvaney]

pugsql modules don't have a set_engine method, but a setengine method. This edit updates the docs to reflect that.

>>> import pugsql
>>> pugsql.__version__
'0.2.3'
>>> queries = pugsql.module('sql')
>>> queries.set_engine
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Module' object has no attribute 'set_engine'
>>> queries.setengine
<bound method Module.setengine of <pugsql.compiler.Module object at 0x102798358>>

[mcfunley]

Thank you!