[docs][llvm-nm] Make --help help text consistent with other options

llvm-svn: 363073
diff --git a/llvm/docs/CommandGuide/llvm-nm.rst b/llvm/docs/CommandGuide/llvm-nm.rst
index f679a04..99e7f18 100644
--- a/llvm/docs/CommandGuide/llvm-nm.rst
+++ b/llvm/docs/CommandGuide/llvm-nm.rst
@@ -103,7 +103,7 @@
  Select an output format; *format* may be *sysv*, *posix*, or *bsd*.  The default
  is *bsd*.
 
-.. option:: -help
+.. option:: --help, -h
 
  Print a summary of command-line options and their meanings.