fix `desc()` syntax

[romainfrancois]

We're about to release dplyr 1.0.8 and we have identified an issue with one of this package vignette.

# sfnetworks

<details>

* Version: 0.5.4
* GitHub: https://github.com/luukvdmeer/sfnetworks
* Source code: https://github.com/cran/sfnetworks
* Date/Publication: 2021-12-17 09:00:02 UTC
* Number of recursive dependencies: 139

Run `cloud_details(, "sfnetworks")` for more info

</details>

## Newly broken

*   checking re-building of vignette outputs ... WARNING

Error(s) in re-building vignettes:
--- re-building ‘sfn01_structure.Rmd’ using rmarkdown
Linking to GEOS 3.8.0, GDAL 3.0.4, PROJ 6.3.1; sf_use_s2() is TRUE

Attaching package: 'tidygraph'

The following object is masked from 'package:stats':

    filter

...
Quitting from lines 190-198 (sfn05_morphers.Rmd) 
Error: processing vignette 'sfn05_morphers.Rmd' failed with diagnostics:
[1m[22m`desc()` must be called with exactly one argument.
--- failed re-building ‘sfn05_morphers.Rmd’

SUMMARY: processing the following file failed:
  ‘sfn05_morphers.Rmd’

Error: Vignette re-building failed.
Execution halted
```

Please consider this fix. 

[codecov-commenter]

Codecov Report

Merging #202 (3e67b11) into master (28cbfb8) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #202   +/-   ##
=======================================
  Coverage   72.16%   72.16%           
=======================================
  Files          21       21           
  Lines        1351     1351           
=======================================
  Hits          975      975           
  Misses        376      376           

---

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[agila5]

Thank you very much. I'm working on the CRAN submission.