[gdb/doc] Use more lower-case in @sc in the documentation

When building gdb with an older makeinfo (4.13), I run into:
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gdb/doc/gdb.texinfo:49064: warning: @sc argument all uppercase, thus no effect.
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Using a grep, I found one more instance:
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$ grep @sc gdb/doc/*.tex* | egrep -v  '@sc{[^A-Z]*}'
gdb/doc/gdb.texinfo:\
Bit 1 (@sc{ZA}) shows whether the @code{ZA} register state is active (in use) or
gdb/doc/python.texi:\
corresponding @sc{GDB/MI} command's output.  Refer to the
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Fix this by using lowercase letters in the @sc argument, similar to how that
was done in commit c96452ad168 ("Use lower-case in @sc in the documentation").

Tested by rebuilding the documentation.