[ptr.align] Use math font 'X' for the object (#7367)

source/memory.tex

@@ -839,9 +839,9 @@
 
 \pnum
 \expects
-\tcode{ptr} points to an object \tcode{X} of
+\tcode{ptr} points to an object $X$ of
 a type similar\iref{conv.qual} to \tcode{T},
-where \tcode{X} has alignment \tcode{N}\iref{basic.align}.
+where $X$ has alignment \tcode{N}\iref{basic.align}.
 
 \pnum
 \returns
@@ -853,13 +853,13 @@
 
 \pnum
 \begin{note}
-The alignment assumption on an object \tcode{X}
+The alignment assumption on an object $X$
 expressed by a call to \tcode{assume_aligned}
 might result in generation of more efficient code.
 It is up to the program to ensure that the assumption actually holds.
 The call does not cause the implementation to verify or enforce this.
 An implementation might only make the assumption
-for those operations on \tcode{X} that access \tcode{X}
+for those operations on $X$ that access $X$
 through the pointer returned by \tcode{assume_aligned}.
 \end{note}
 \end{itemdescr}