[DOCS][10MINS] Fix method for the 'where' operation example
Fix method of the 'where' operation for getting.

@moutai This is in the 'Boolean indexing' section, so I think it is on purpose that it is not using `where`.
 But you could e.g. add something like "which is equivalent to `df.where(df > 0)`"
 I see, that makes sense in the Boolean indexing section. Please ignore.
