[forth/en] correct some mistakes, make better wording in some places

[ruv]

Changes

• A variable size cannot be changed in Forth (see variables).
• variable shall not be used to create a variable that holds a floating-point number. Use fvariable instead.
• In Forth, i is NOT equivalent to r@. Remove the corresponding sentence.
• Add an example of using the return stack and begin ... while ... repeat loop.
• Make one-to-12 do what its name suggests.
• Add a link to the Forth Standard.
• Make some comments more consistent (e.g. instead of using the terms "item" and "element", use "item" throughout).
• In some places, make comments more correct or detailed, briefly explain the stack diagrams.

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• I solemnly swear that this is all original content of which I am the original author
• Pull request title is prepended with [language/lang-code] (example [python/fr-fr] or [java/en])
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• Content changes are aimed at intermediate to experienced programmers (this is a poor format for explaining fundamental programming concepts)
• The YAML Frontmatter was not changed.