Quote `interrogate[png]` in installation instructions

[johnslavik]

Hey, I just made a Pull Request!

Description

Quoted interrogate[png] in pip install interrogate[png].

Motivation and Context

In Z shell, square brackets trigger pattern matching. That may cause this to happen:

$ pip install interrogate[png]
zsh: no matches found: interrogate[png]

The quoted version is the most universal.