Revert changes to legal docs and British English spelling

Revert copyright.rst, license.rst (legal documents should not be
modified), and inputoutput.rst (British spelling "behaviour" is
acceptable in CPython docs).

Author: him2him2

Doc/copyright.rst

@@ -2,7 +2,7 @@
 Copyright
 *********
 
-Python and this documentation are:
+Python and this documentation is:
 
 Copyright © 2001 Python Software Foundation. All rights reserved.
 

Doc/license.rst

@@ -599,7 +599,7 @@ interface::
 SipHash24
 ---------
 
-The file :file:`Python/pyhash.c` contains Marek Majkowski's implementation of
+The file :file:`Python/pyhash.c` contains Marek Majkowski' implementation of
 Dan Bernstein's SipHash24 algorithm. It contains the following note::
 
   <MIT License>

Doc/tutorial/inputoutput.rst

@@ -476,7 +476,7 @@ In text files (those opened without a ``b`` in the mode string), only seeks
 relative to the beginning of the file are allowed (the exception being seeking
 to the very file end with ``seek(0, 2)``) and the only valid *offset* values are
 those returned from the ``f.tell()``, or zero. Any other *offset* value produces
-undefined behavior.
+undefined behaviour.
 
 File objects have some additional methods, such as :meth:`~io.IOBase.isatty` and
 :meth:`~io.IOBase.truncate` which are less frequently used; consult the Library