Langmuir fit doc update

[Surluson]

Added docs example for fit_adsorption_isotherm

Removed trailing end-line characters from markdown files (which is why Github thinks I changed everything)

removed deprecation warning from PengRobinsonFluid (The example is up-to-date on the latest docs. I think we just need to update the PorousMaterials version (?))

[coveralls]

Pull Request Test Coverage Report for Build 607

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 80.222%

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Totals
Change from base Build 576:	0.1%
Covered Lines:	1156
Relevant Lines:	1441

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💛 - Coveralls