typo in docs for pdfrw fork link fixed

[semaeostomea]

This just fixes a tiny typo in the docs that made the link to install from the pdfrw fork invalid

Checklist:

• The GitHub pipeline is OK (green),
  meaning that both pylint (static code analyzer) and black (code formatter) are happy with the changes of this PR.

• This PR is ready to be merged

By submitting this pull request, I confirm that my contribution is made under the terms of the GNU LGPL 3.0 license.

[codecov]

Codecov Report

Merging #522 (4699878) into master (25b9ffb) will not change coverage.
The diff coverage is n/a.

❗ Current head 4699878 differs from pull request most recent head 6aa39a6. Consider uploading reports for the commit 6aa39a6 to get more accurate results

@@           Coverage Diff           @@
##           master     #522   +/-   ##
=======================================
  Coverage   92.34%   92.34%           
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  Files          23       23           
  Lines        6860     6860           
  Branches     1405     1405           
=======================================
  Hits         6335     6335           
  Misses        299      299           
  Partials      226      226           

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[Lucas-C]

Thank you for this fix!
Merging now

[Lucas-C]

@all-contributors please add @semaeostomea for documentation

[allcontributors]

@Lucas-C

@semaeostomea already contributed before to doc