Fix typo in "PostgreSQL" in project description

Author: garyd203

setup.py

@@ -246,7 +246,7 @@ def finalize_options(self):
 setuptools.setup(
     name='asyncpg',
     version=VERSION,
-    description='An asyncio PosgtreSQL driver',
+    description='An asyncio PostgreSQL driver',
     long_description=readme,
     classifiers=[
         'Development Status :: 5 - Production/Stable',