Docs: fix typos of 'whether' in operator_{gt,le,lt}.md

[tsnl]

Fixed some typos in the documentation that I noticed while browsing the docs on operator<.

The word "whether" was typo-ed as "wether." I fixed this typo and all others I could find in the repo using simple text-search after confirming each one.

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• Changes are described in the pull request, or an existing issue is referenced.
• The test suite compiles and runs without error.
• Code coverage is 100%. Test cases can be added by editing the test suite.
• The source code is amalgamated; that is, after making changes to the sources in the include/nlohmann directory, run make amalgamate to create the single-header files single_include/nlohmann/json.hpp and single_include/nlohmann/json_fwd.hpp. The whole process is described here.

[nlohmann]

Looks good to me.

[nlohmann]

Thanks!