improve help string for --dep-graph

The current one is not clear enough as to what `<ext>` is supposed to be.
So provide some examples, especially `.dot` which doesn't require `graphviz-python`

easybuild/tools/options.py

@@ -656,7 +656,8 @@ def informative_options(self):
             'check-conflicts': ("Check for version conflicts in dependency graphs", None, 'store_true', False),
             'check-eb-deps': ("Check presence and version of (required and optional) EasyBuild dependencies",
                               None, 'store_true', False),
-            'dep-graph': ("Create dependency graph", None, 'store', None, {'metavar': 'depgraph.<ext>'}),
+            'dep-graph': ("Create dependency graph. Output format depends on <ext>, e.g. 'dot', 'png', 'pdf', 'gv'.",
+                          None, 'store', None, {'metavar': 'depgraph.<ext>'}),
             'dump-env-script': ("Dump source script to set up build environment based on toolchain/dependencies",
                                 None, 'store_true', False),
             'last-log': ("Print location to EasyBuild log file of last (failed) session", None, 'store_true', False),