fix typo in developer_guide.md

[unbtorsten]

Description

Adds a missing equal sign to pin the version of clang.

How to test this?

Create virtual environment according to current guide...

$ pip install clang-format=12.0.1
ERROR: Invalid requirement: 'clang-format=12.0.1'
Hint: = is not a valid operator. Did you mean == ?

...versus running it according to the fixed guide:

$ pip install clang-format==12.0.1
Collecting clang-format==12.0.1

[pramodk]

thanks @unbtorsten !