FAQ: fix error message

The package is called bcc, and not bpf:

    from bcc import BPF

If that is not available the error message reads

    ModuleNotFoundError: No module named 'bcc'

and not `No module named 'bpf'`

FAQ.txt

@@ -11,7 +11,7 @@ A: make sure to 'make install' and add the directory
    export LD_LIBRARY_PATH=/usr/local/lib:$LD_LIBRARY_PATH
 
 Q: hello_world.py fails with:
-   ImportError: No module named bpf
+   ImportError: No module named bcc
 A: checkout "sudo make install" output to find out bpf package installation site,
    add it to the PYTHONPATH env variable before running the program.
    sudo bash -c 'PYTHONPATH=/usr/lib/python2.7/site-packages python examples/hello_world.py'