[cpp.predefined] Fix index entries

Author: tkoeppe

source/preprocessor.tex

@@ -1753,15 +1753,15 @@
 as an extended floating-point type.
 
 \item
-\indextext{__stdcpp_float16_t__@\mname{STDCPP_FLOAT128_T}}%
+\indextext{__stdcpp_float128_t__@\mname{STDCPP_FLOAT128_T}}%
 \mname{STDCPP_FLOAT128_T}\\
 Defined as the integer literal \tcode{1}
 if and only if the implementation supports
 the ISO/IEC/IEEE 60559 floating-point interchange format binary128
 as an extended floating-point type.
 
 \item
-\indextext{__stdcpp_float16_t__@\mname{STDCPP_BFLOAT16_T}}%
+\indextext{__stdcpp_bfloat16_t__@\mname{STDCPP_BFLOAT16_T}}%
 \mname{STDCPP_BFLOAT16_T}\\
 Defined as the integer literal \tcode{1}
 if and only if the implementation supports an extended floating-point type