docs(numpy-mini-project): fix typos and grammar in mean normalization notebook

[computationalcore]

What

• Fix typos in the “Mean Normalization and Data Separation” notebook (and Solution), including: “Rememeber” → “Remember”, “machine learnig” → “machine learning”, “filing” → “filling”, and “X_tain” → “X_train”.
• Minor grammar/clarity edits (e.g., “between -3 and 3”, “the average”, “have created”) with no changes to code or notebook behavior.