help: prepend environment variables used in hooks with "$"

Most of them have "$" prefix but some don't, so adding it for consistency.

Differential Revision: https://phab.mercurial-scm.org/D10398

mercurial/helptext/config.txt

@@ -1151,7 +1151,7 b' be ``$HG_HOOKTYPE=incoming`` and ``$HG_H'
 ``pretxnopen``
   Run before any new repository transaction is open. The reason for the
   transaction will be in ``$HG_TXNNAME``, and a unique identifier for the
-  transaction will be in ``HG_TXNID``. A non-zero status will prevent the
+  transaction will be in ``$HG_TXNID``. A non-zero status will prevent the
   transaction from being opened.
 
 ``pretxnclose``
@@ -1160,12 +1160,12 b' be ``$HG_HOOKTYPE=incoming`` and ``$HG_H'
   content or change it. Exit status 0 allows the commit to proceed. A non-zero
   status will cause the transaction to be rolled back. The reason for the
   transaction opening will be in ``$HG_TXNNAME``, and a unique identifier for
-  the transaction will be in ``HG_TXNID``. The rest of the available data will
+  the transaction will be in ``$HG_TXNID``. The rest of the available data will
   vary according the transaction type. New changesets will add ``$HG_NODE``
   (the ID of the first added changeset), ``$HG_NODE_LAST`` (the ID of the last
   added changeset), ``$HG_URL`` and ``$HG_SOURCE`` variables.  Bookmark and
-  phase changes will set ``HG_BOOKMARK_MOVED`` and ``HG_PHASES_MOVED`` to ``1``
-  respectively, etc.
+  phase changes will set ``$HG_BOOKMARK_MOVED`` and ``$HG_PHASES_MOVED`` to
+  ``1`` respectively, etc.
 
 ``pretxnclose-bookmark``
   Run right before a bookmark change is actually finalized. Any repository
@@ -1179,7 +1179,7 b' be ``$HG_HOOKTYPE=incoming`` and ``$HG_H'
   will be empty.
   In addition, the reason for the transaction opening will be in
   ``$HG_TXNNAME``, and a unique identifier for the transaction will be in
-  ``HG_TXNID``.
+  ``$HG_TXNID``.
 
 ``pretxnclose-phase``
   Run right before a phase change is actually finalized. Any repository change
@@ -1191,7 +1191,7 b' be ``$HG_HOOKTYPE=incoming`` and ``$HG_H'
   while the previous ``$HG_OLDPHASE``. In case of new node, ``$HG_OLDPHASE``
   will be empty.  In addition, the reason for the transaction opening will be in
   ``$HG_TXNNAME``, and a unique identifier for the transaction will be in
-  ``HG_TXNID``. The hook is also run for newly added revisions. In this case
+  ``$HG_TXNID``. The hook is also run for newly added revisions. In this case
   the ``$HG_OLDPHASE`` entry will be empty.
 
 ``txnclose``