[Reference Folder]: Spelling

[BethanyG]

Because this seems to be a perennial problem with eager contributors, I went ahead and corrected the spelling.
I will also try to clarify going forward that the reference folder is maintainer-facing, and largely off limits for editing/change/contribution.

The following files were changed:

• ../reference/concepts/boolean_values.md
• ../reference/concepts/builtin_types/dict.md
• ../reference/concepts/builtin_types/frozenset.md
• ../reference/concepts/builtin_types/list.md
• ../reference/concepts/constructor.md
• ../reference/concepts/default_arguments.md
• ../reference/concepts/dunder_methods.md
• ../reference/concepts/initialization.md
• ../reference/concepts/instance_attributes.md
• ../reference/concepts/instance_methods.md
• ../reference/concepts/instance_properties.md
• ../reference/concepts/regular_expressions.md
• ../reference/concepts/return_value.md
• ../reference/concepts/slicing.md
• ../reference/concepts/string_splitting.md
• ../reference/concepts/string_translation.md
• ../reference/concepts/type_hinting.md
• ../reference/exercise-concepts/binary-search-tree.md
• ../reference/exercise-concepts/leap.md
• ../reference/exercise-concepts/markdown.md
• ../reference/exercise-concepts/matrix.md
• ../reference/exercise-concepts/phone-number.md
• ../reference/exercise-concepts/reverse-string.md
• ../reference/exercise-concepts/rna-transcription.md
• ../reference/exercise-concepts/robot-simulator.md