DATAMONGO-1508 - Polishing.

Highlight attribute name. Replace tabs with spaces.

Original pull request: #399.

src/main/asciidoc/reference/mongodb.adoc

@@ -387,7 +387,7 @@ You can also provide the host and port for the underlying `com.mongodb.Mongo` in
                   password="secret"/>
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-If your mongodb authentication database differs from the target database, use the authentication-dbname attribute, as shown below.
+If your MongoDB authentication database differs from the target database, use the `authentication-dbname` attribute, as shown below.
 
 [source,xml]
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