Fix documentation errors

[SeanBryan51]

Fixes the following errors:

• In user_guide.md - 'Directory structure and files', tasks should be runs/fluxsite/tasks.
• In config_options.md - "NRI Land testing" should be "benchcab-evaluation"
• In internal.py - "NRI Land testing" should be "benchcab-evaluation"

Fixes #131

[codecov]

Codecov Report

Merging #132 (41d1dc6) into master (093bedf) will not change coverage.
Report is 1 commits behind head on master.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #132   +/-   ##
=======================================
  Coverage   88.47%   88.47%           
=======================================
  Files          27       27           
  Lines        1527     1527           
=======================================
  Hits         1351     1351           
  Misses        176      176           

Files Changed	Coverage Δ
benchcab/internal.py	90.24% <ø> (ø)

[ccarouge]

A minor suggestion for the documentation.