[basic.extended.fp] Replaced usage of 'mantissa' with 'significand'

according to SO/IEC/IEEE 60559:2008

Author: MFHava

source/basic.tex

@@ -5248,8 +5248,8 @@
 \pnum
 \begin{note}
 A summary of the parameters for each type is given in \tref{basic.extended.fp}.
-The precision $p$ includes the implicit 1 bit at the beginning of the mantissa,
-so the storage used for the mantissa is $p-1$ bits.
+The precision $p$ includes the implicit 1 bit at the beginning of the significand,
+so the storage used for the significand is $p-1$ bits.
 ISO/IEC/IEEE 60559 does not assign a name for a type
 having the parameters specified for \tcode{std::bfloat16_t}.
 \end{note}