Approximating square root (loops and functions exercise)
One tiny thing: If you drop one set of parentheses:

return z - ((z * z - x) / (2 * z))


and then run go fmt, it will change it to

return z - ((z*z - x) / (2 * z))


The expressions around the * are close to the * and the expressions around the - are spaced out. In other words, go fmt implies the correct precedence.

Also, using line breaks to separate "sections" of a function is good, but there's no need to add an extra linebreak at the very beginning of a function or just before a }.