[Fix] Fix fastscnn path

[MengzhangLI]

Motivation:
Fix fastscnn config link, it is wrong right now.

Modification
Change link from
https://github.com/open-mmlab/mmsegmentation/blob/master/configs/fast_scnn.py
to
https://github.com/open-mmlab/mmsegmentation/blob/master/configs/fastscnn/fast_scnn_lr0.12_8x4_160k_cityscapes.py

[codecov]

Codecov Report

Merging #760 (7f4fa9b) into master (6394679) will not change coverage.
The diff coverage is n/a.

❗ Current head 7f4fa9b differs from pull request most recent head 0f66a7f. Consider uploading reports for the commit 0f66a7f to get more accurate results

@@           Coverage Diff           @@
##           master     #760   +/-   ##
=======================================
  Coverage   85.37%   85.37%           
=======================================
  Files         107      107           
  Lines        5847     5847           
  Branches      960      960           
=======================================
  Hits         4992     4992           
  Misses        673      673           
  Partials      182      182           

Flag	Coverage Δ
unittests	85.36% <ø> (ø)

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[Junjun2016]

LGTM