Fix NameError in 'Evaluation order matters'

This code in 'Indexing and selecting data' of doc wouldn't execute

Author: sergei3000

doc/source/user_guide/indexing.rst

@@ -1837,7 +1837,7 @@ This however is operating on a copy and will not work.
    :okwarning:
    :okexcept:
 
-   with option_context('mode.chained_assignment','warn'):
+   with pd.option_context('mode.chained_assignment','warn'):
        dfb[dfb['a'].str.startswith('o')]['c'] = 42
 
 A chained assignment can also crop up in setting in a mixed dtype frame.