bpo-44713: [doc fix]: typo in subprocess.rst (GH-27297)

This fixes a small typo. The code fragment should not be quoted. Thank you
@merwok for the feedback.

Author: jdevries3133

Doc/library/subprocess.rst

@@ -689,7 +689,7 @@ execute, will be re-raised in the parent.
 
 The most common exception raised is :exc:`OSError`.  This occurs, for example,
 when trying to execute a non-existent file.  Applications should prepare for
-:exc:`OSError` exceptions. Note that, when ``"shell=True"``, :exc:`OSError`
+:exc:`OSError` exceptions. Note that, when ``shell=True``, :exc:`OSError`
 will be raised by the child only if the selected shell itself was not found.
 To determine if the shell failed to find the requested application, it is
 necessary to check the return code or output from the subprocess.