Fix parameter path bug in windows

[Saransh-cpp]

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.

Fixes #1804

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #) - note reverse order of PR #s. If necessary, also add to the list of breaking changes.

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist:

• No style issues: $ flake8
• All tests pass: $ python run-tests.py --unit
• The documentation builds: $ cd docs and then $ make clean; make html

You can run all three at once, using $ python run-tests.py --quick.

Further checks:

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

Merging #1900 (934336c) into develop (25ceb5f) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1900   +/-   ##
========================================
  Coverage    99.28%   99.28%           
========================================
  Files          345      345           
  Lines        19108    19116    +8     
========================================
+ Hits         18972    18980    +8     
  Misses         136      136           

Impacted Files	Coverage Δ
pybamm/parameters/parameter_values.py	99.46% <100.00%> (+<0.01%)	⬆️
pybamm/util.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 25ceb5f...934336c. Read the comment docs.

[valentinsulzer]

Thanks