help: glossary.txt formatting fixes

mercurial/help/glossary.txt

@@ -151,7 +151,7 b' DAG'
     consisting of nodes and edges, where nodes correspond to
     changesets and edges imply a parent -> child relation. This graph
     can be visualized by graphical tools such as :hg:`glog`
-    (graphlog). In mercurial, the DAG is limited by the requirement
+    (graphlog). In Mercurial, the DAG is limited by the requirement
     for children to have at most two parents.
 
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