[RTM] update readme with external forces exemple

[Ipuch]

and fix the previous exemple.

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This change is 

[pariterre]

Reviewed 2 of 2 files at r2, 1 of 1 files at r3, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @Ipuch)

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README.md line 128 at r3 (raw file):

import numpy as np
import bioviz
manually_animate = False

This is not used

Code quote:

manually

[pariterre]

Reviewed 1 of 1 files at r4, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @Ipuch)

[Ipuch]

@pariterre mergeable ?