bpo-45799: [Doc] improve confusing sentence in __main__.rst (GH-29546)

I was reading this bit last night and thought it was a typo. In the light of day, I realized it wasn't *technically* a typo, but definitely confusing wording. This PR fixes the confusing sentence.

Automerge-Triggered-By: GH:ericvsmith

Author: jdevries3133

Doc/library/__main__.rst

@@ -116,8 +116,8 @@ Idiomatic Usage
 ^^^^^^^^^^^^^^^
 
 Some modules contain code that is intended for script use only, like parsing
-command-line arguments or fetching data from standard input.  When a module
-like this were to be imported from a different module, for example to unit test
+command-line arguments or fetching data from standard input.  If a module
+like this was imported from a different module, for example to unit test
 it, the script code would unintentionally execute as well.
 
 This is where using the ``if __name__ == '__main__'`` code block comes in