Add missing slash in selecting dynamically-loaded content documentation

[dashtinejad]

I've been reading scrapy documentation and I've realized that a slash is missing in one of the examples. This PR adds the missing slash in the "selecting dynamically-loaded content" documentation.

[codecov]

Codecov Report

Merging #5890 (48a98bb) into master (c34ca4a) will decrease coverage by 0.09%.
The diff coverage is 86.99%.

❗ Current head 48a98bb differs from pull request most recent head a71eadb. Consider uploading reports for the commit a71eadb to get more accurate results

@@            Coverage Diff             @@
##           master    #5890      +/-   ##
==========================================
- Coverage   88.94%   88.85%   -0.09%     
==========================================
  Files         162      162              
  Lines       11002    11057      +55     
  Branches     1798     1801       +3     
==========================================
+ Hits         9786     9825      +39     
- Misses        937      954      +17     
+ Partials      279      278       -1     

Impacted Files	Coverage Δ
scrapy/commands/__init__.py	74.71% <ø> (ø)
scrapy/commands/bench.py	100.00% <ø> (ø)
scrapy/commands/crawl.py	60.00% <ø> (ø)
scrapy/commands/edit.py	51.85% <ø> (ø)
scrapy/commands/fetch.py	89.13% <ø> (ø)
scrapy/commands/list.py	77.77% <ø> (ø)
scrapy/commands/runspider.py	93.47% <ø> (ø)
scrapy/commands/settings.py	71.87% <ø> (ø)
scrapy/commands/shell.py	92.85% <ø> (ø)
scrapy/commands/startproject.py	100.00% <ø> (ø)
... and 46 more

... and 1 file with indirect coverage changes

[Gallaecio]

Thanks!