docs: Fix uos and utime heading underlines to be the correct length.

Otherwise Sphinx gives a warning.

docs/library/uos.rst

 :mod:`uos` -- basic "operating system" services
-==============================================
+===============================================
 
 .. module:: uos
    :synopsis: basic "operating system" services

docs/library/utime.rst

 :mod:`utime` -- time related functions
-=====================================
+======================================
 
 .. module:: utime
    :synopsis: time related functions