Fix wget syntax in documentation

[Lusitaniae]

The syntax was wrong and doesn't work as expected

       -O file
       --output-document=file
           The documents will not be written to the appropriate files, but all will be concatenated together and written to file.  If - is used as file, documents
           will be printed to standard output, disabling link conversion.  (Use ./- to print to a file literally named -.)

Capital "O" solves it.

[claassistantio]

All committers have signed the CLA.

[codecov-io]

Codecov Report

Merging #1609 into master will increase coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1609      +/-   ##
==========================================
+ Coverage   60.69%   60.75%   +0.06%     
==========================================
  Files         107      107              
  Lines        8075     8075              
==========================================
+ Hits         4901     4906       +5     
+ Misses       2787     2784       -3     
+ Partials      387      385       -2

Impacted Files	Coverage Δ
pkg/promtail/targets/filetarget.go	70.55% <0%> (+1.84%)	⬆️
pkg/promtail/targets/tailer.go	75.86% <0%> (+2.29%)	⬆️

[cyriltovena]

LGTM