Replace Ti with $\tcode{T}_i$ for proper indexes.

And similar for Ui, Vi, and TiD; the latter becoming $\tcode{TD}_i$.

Partially addresses cplusplus#1139.

source/threads.tex

@@ -518,7 +518,7 @@
 
 \begin{itemdescr}
 \pnum
-\requires\ \tcode{F} and each \tcode{Ti} in \tcode{Args} shall satisfy the
+\requires\ \tcode{F} and each $\tcode{T}_i$ in \tcode{Args} shall satisfy the
 \tcode{MoveConstructible} requirements.
 \tcode{%
 \placeholdernc{INVOKE}(\brk{}%
@@ -4809,7 +4809,7 @@
 
 \begin{itemdescr}
 \pnum
-\requires \tcode{F} and each \tcode{Ti} in \tcode{Args} shall
+\requires \tcode{F} and each $\tcode{T}_i$ in \tcode{Args} shall
 satisfy the
 \tcode{MoveConstructible} requirements, and
 \begin{codeblock}

source/utilities.tex

@@ -1259,10 +1259,10 @@
 \begin{itemdescr}
 \pnum
 \returns \tcode{pair<V1, V2>(std::forward<T1>(x), std::forward<T2>(y))},
-where \tcode{V1} and \tcode{V2} are determined as follows: Let \tcode{Ui} be
-\tcode{decay_t<Ti>} for each \tcode{Ti}. If \tcode{Ui} is a specialization
-of \tcode{reference_wrapper}, then \tcode{Vi} is \tcode{Ui::type\&},
-otherwise \tcode{Vi} is \tcode{Ui}.
+where \tcode{V1} and \tcode{V2} are determined as follows: Let $\tcode{U}_i$ be
+\tcode{decay_t<$\tcode{T}_i$>} for each $\tcode{T}_i$. If $\tcode{U}_i$ is a specialization
+of \tcode{reference_wrapper}, then $\tcode{V}_i$ is \tcode{$\tcode{U}_i$::type\&},
+otherwise $\tcode{V}_i$ is $\tcode{U}_i$.
 \end{itemdescr}
 
 \pnum