[thread.req.timing] Use mathematical symbols

Author: tkoeppe

source/threads.tex

@@ -104,8 +104,8 @@
 duration of the wait decreasing when measured against a steady clock. \end{note}
 
 \item
-if $C_a <= C_t$, the waiting function should not time out until \tcode{Clock::now()} returns a
-time $C_n >= C_t$, i.e. waking at $C_t + D_i + D_m$. \begin{note} When the clock is adjusted
+if $C_a \leq C_t$, the waiting function should not time out until \tcode{Clock::now()} returns a
+time $C_n \geq C_t$, i.e. waking at $C_t + D_i + D_m$. \begin{note} When the clock is adjusted
 backwards, this specification may result in the total duration of the wait increasing when
 measured against a steady clock. When the clock is adjusted forwards, this specification may
 result in the total duration of the wait decreasing when measured against a steady clock.