We say 'a constant .. expression' elsewhere. Fix the grammar.
gcc/ChangeLog:
* doc/extend.texi: Add 'a' for grammar fix.
This function has the same semantics as @code{__builtin_expect},
but the caller provides the expected probability that @var{exp} == @var{c}.
The last argument, @var{probability}, is a floating-point value in the
-range 0.0 to 1.0, inclusive. The @var{probability} argument must be
+range 0.0 to 1.0, inclusive. The @var{probability} argument must be a
constant floating-point expression.
@enddefbuiltin