Flexbox - Growing and Shrinking lesson: update flex shorthand section for clarity with note on default values

[DanielKolocka]

Because

This pull request clarifies the explanation of the flex shorthand property by explicitly showing its default values (flex-grow: 0, flex-shrink: 1, flex-basis: auto). It resolves the confusion around why flex: 1 results in flex-grow: 1, flex-shrink: 1, and flex-basis: 0 by making the default behavior clear. This update helps users better understand how the flex shorthand works and how default values impact the layout.

This PR

• Added clarification about the default values of the flex shorthand (flex-grow: 0, flex-shrink: 1, flex-basis: auto).
• Explained how flex: 1 equates to flex-grow: 1, flex-shrink: 1, and flex-basis: 0.

Issue

Closes #XXXXX

Additional Information

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