The second example at https://www.gnu.org/software/sed/ is as follows:

# Example: replace every occurrence of 'hello' with 'world' on lines 10-20
$ sed '10,20s/hello/world/' input.txt > output.txt

That sed command does not do what the comment says it does.

For example, if line 10 of input.txt is `hello hello` the output for that line is `world hello`.

Perhaps add a g switch after the third slash, or alternately, change the description to something like "on each of lines 10-20, replace the first occurrence (if any) of 'hello' with 'world'" or,