help/config: reword committemplate external editor text

mercurial/help/config.txt

@@ -449,9 +449,10 For example, ``changeset.tag.remove`` cu
 only for :hg:`tag --remove`, but ``changeset.tag`` customizes the
 commit message for :hg:`tag` regardless of ``--remove`` option.
 
-At the external editor invocation for committing, corresponding
-dot-separated list of names without ``changeset.`` prefix
-(e.g. ``commit.normal.normal``) is in ``HGEDITFORM`` environment variable.
+When the external editor is invoked for a commit, the corresponding
+dot-separated list of names without the ``changeset.`` prefix
+(e.g. ``commit.normal.normal``) is in the ``HGEDITFORM`` environment
+variable.
 
 In this section, items other than ``changeset`` can be referred from
 others. For example, the configuration to list committed files up