doc: Minor fixes in safegcd_implementation.md #1010

 575 | @@ -576,7 +576,7 @@ i = N # divsteps left to do
 576 |  while True:
 577 |      # Get rid of all bottom zeros at once. In the first iteration, g may be odd and the following
 578 |      # lines have no effect (until "if eta < 0").
 579 | -    zeros = min(i, count_trailing_zeros(g))
 580 | +    zeros = i if g == 0 else min(i, count_trailing_zeros(g))

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 569 | @@ -570,7 +570,10 @@ bits efficiently, which is possible on most platforms; it is abstracted here as
 570 |  ```python
 571 |  def count_trailing_zeros(v):
 572 |      """For a non-zero value v, find z such that v=(d<<z) for some odd d."""

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