help: uppercase command placeholder

'command' isn't a valid help topic but a placeholder text. Make it upper
case to avoid confusion.

hgext/mq.py

@@ -14,7 +14,7 b' applied patches (subset of known patches'
 Known patches are represented as patch files in the .hg/patches
 directory. Applied patches are both patch files and changesets.
 
-Common tasks (use :hg:`help command` for more details)::
+Common tasks (use :hg:`help COMMAND` for more details)::
 
   create new patch                          qnew
   import existing patch                     qimport

tests/test-mq.t

@@ -25,7 +25,7 b' help'
   Known patches are represented as patch files in the .hg/patches directory.
   Applied patches are both patch files and changesets.
   
-  Common tasks (use 'hg help command' for more details):
+  Common tasks (use 'hg help COMMAND' for more details):
   
     create new patch                          qnew
     import existing patch                     qimport