docs: Improved error message of encode_string

[frgfm]

This PR improves the error message of encode_string when a character is not to be found in the vocab.

Closes #927

Any feedback is welcome!

[frgfm]

FYI, there is already a test checking that this case throws a ValueError (and I don't think we need to check the error message additionally 😅 )

[codecov]

Codecov Report

Merging #929 (7be9cc2) into main (607aaaf) will decrease coverage by 0.01%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##             main     #929      +/-   ##
==========================================
- Coverage   94.71%   94.70%   -0.02%     
==========================================
  Files         134      134              
  Lines        5488     5491       +3     
==========================================
+ Hits         5198     5200       +2     
- Misses        290      291       +1     

Flag	Coverage Δ
unittests	94.70% <50.00%> (-0.02%)	⬇️

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Impacted Files	Coverage Δ
doctr/datasets/utils.py	93.33% <50.00%> (-2.50%)	⬇️
doctr/transforms/functional/base.py	97.10% <0.00%> (+1.44%)	⬆️

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[felixdittrich92]

@frgfm
Thanks LGTM :)
But next time i would recommend to give maybe up to 2 weeks for possible new contributors to solve some easy tasks 😅

[frgfm]

Yes I hesitated, but I figured no one had shown interest initially, but I'll keep that in mind next time 👍

Actually @fharper, I've seen a few OSS libraries keeping a few issues on the side for newcomers, and posting it on social media (and they got new contributors with that). That could be an idea 🤷

[felixdittrich92]

@frgfm @fharper
That sounds like a good idea :)
Should we open a discussion to talk about some easy to solve tasks ? Or maybe better switch to Slack ?

[fharper]

Let's move the discussion on Slack, so we don't spam people here.