[3.8] swapped scalar 1 by identity matrix in grover.ipynb

[HannesGitH]

[3.8] use I instead of 1 for the identity matrix (since subtracting a scalar from a matrix is undefined)

Changes made

Swapped \\mathbb{1} with I in following eqaution:

- U = 2 |s> <s| - 1
+ U = 2 |s> <s| - I

Justification

2 |s> <s| is a matrix, and its mathematically not clear/defined how to substract a scalar (1) from this matrix, using I which is commonly used as the identity matrix is better

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