[FileCheck][Docs] Fix regex for FileCheck variable names (#97301)

This fixes a minor oversight in the FileCheck documentation on what is
considered a valid variable name.

Global variables are prefixed with a `$`, which is explained two
paragraphs below, but this was omitted in the presented regex in this
paragraph.

Author: AntonLydike

llvm/docs/CommandGuide/FileCheck.rst

@@ -731,7 +731,7 @@ The first check line matches a regex ``%[a-z]+`` and captures it into the
 string variable ``REGISTER``.  The second line verifies that whatever is in
 ``REGISTER`` occurs later in the file after an "``andw``". :program:`FileCheck`
 string substitution blocks are always contained in ``[[ ]]`` pairs, and string
-variable names can be formed with the regex ``[a-zA-Z_][a-zA-Z0-9_]*``.  If a
+variable names can be formed with the regex ``\$[a-zA-Z_][a-zA-Z0-9_]*``.  If a
 colon follows the name, then it is a definition of the variable; otherwise, it
 is a substitution.