Index and whitespace fixes for doc/misc/sc.texi.

Index and whitespace fixes for doc/misc/sc.texi.

* sc.texi (Introduction): Fix indexes.
(Usage Overview):
(Citations, Citation Elements, Recognizing Citations)
(Information Keys and the Info Alist, Reference Headers)
(The Built-in Header Rewrite Functions)
(Electric References, Reply Buffer Initialization)
(Filling Cited Text, Selecting an Attribution)
(Attribution Preferences)
(Anonymous Attributions, Author Names)
(Using Regi, Post-yank Formatting Commands)
(Citing Commands, Insertion Commands)
(Mail Field Commands)
(Hints to MUA Authors, Thanks and History): Change from one space
between sentences to two.