[docs][llvm-symbolizer] Fix --functions description

"--functions none" and "--functions=none" are not the same. One is the
option "--functions" with its default value of "linkage", followed by an
input address of "none", and the other is "--functions" with the value
"none". This patch fixes the doc to match the actual behaviour by adding
an extra '=' sign in the allowed values description.

llvm/docs/CommandGuide/llvm-symbolizer.rst

@@ -181,7 +181,7 @@ OPTIONS
 
 .. _llvm-symbolizer-opt-f:
 
-.. option:: --functions [<none|short|linkage>], -f
+.. option:: --functions [=<none|short|linkage>], -f
 
   Specify the way function names are printed (omit function name, print short
   function name, or print full linkage name, respectively). Defaults to