Fix typos and improve wording in chapter 4

This MR fixes several typos and improves wording in some areas of chapter 4 to increase clarity.

When reading through paragraph 4.2. The wording of the following snippet was confusing to me.

I have added an explicit auxiliary verb, which I believe conveys the meaning of the sentence a bit more clear.

The snippet now reads:

The value of the inner product <..> indicates the probability amplitude (not the probability) of measuring a system, which is characterised by the state <..>, to be in the state <..>.

This MR also fixes a small typo in the last paragraph of 4.2. The original paragraph:

Furthermore, fixes another small typo and changed wording to improve clarity in 4.3. The original paragraph: |

Also, fixes another small typo in 4.3, as well as removes unnecessary repeated wording. The original paragraph:

Also, fixes small error in subscript in 4.3. The original paragraph: