[format.context] Use \indextext, not \index. (#3179)

Author: Eelis

source/utilities.tex

@@ -20806,7 +20806,7 @@
 \effects Equivalent to: \tcode{out_ = it;}
 \end{itemdescr}
 
-\index{left-pad}%
+\indextext{left-pad}%
 \begin{example}
 \begin{codeblock}
 struct S { int value; };