[basic.fundamental] Fix index entries for sizeof (#1465)

Fixes #1461.

Author: jensmaurer

source/basic.tex

@@ -3660,7 +3660,7 @@
 \indextext{type!fundamental}%
 \indextext{type!integral}%
 \indextext{type!floating-point}%
-\indextext{type!implementation-defined @\tcode{sizeof}}%
+\indextext{type!implementation-defined \tcode{sizeof}}%
 \indextext{type!Boolean}%
 \indextext{type!\idxcode{char}}%
 \indextext{type!character}%
@@ -3721,7 +3721,7 @@
 The standard and
 extended signed integer types are collectively called
 \defnx{signed integer types}{signed integer type}.
-\indextext{integral type!implementation-defined @\tcode{sizeof}}%
+\indextext{integral type!implementation-defined \tcode{sizeof}}%
 Plain
 \tcode{int}s have the natural size suggested by the architecture of the
 execution environment%