[macros] Fix typo in LaTeX comment (#7375)

Author: Andreas-Krug

source/macros.tex

@@ -147,7 +147,7 @@
 
 % \indeximpldef synthesizes a collation key from the argument; that is, an
 % invocation \indeximpldef{arg} emits an index entry `key@arg`, where `key`
-% is derived from `arg` by replacing the folowing list of commands with their
+% is derived from `arg` by replacing the following list of commands with their
 % bare content. This allows, say, collating plain text and code.
 \newcommand{\indeximpldef}[1]{%
 \let\otextup\textup%