doc/rados/operations: Fix invalid hyperlink in crush-map-edits.rst

Fix attempted use of underscores for inline emphasis which resulted in
the text being emphasized to be considered a link. The text was rendered
partially as a link to an invalid anchor "#id3".

Instead use inline italic for formatting emphasis.

Signed-off-by: Ville Ojamo <[email protected]>

Author: bluikko

doc/rados/operations/crush-map-edits.rst

@@ -265,7 +265,7 @@ all of the
 chassis's hosts and their associated OSDs will be in the ``down``
 state and thus unavailable.  It is important to avoid placing multiple replicas
 or shards of data within a single such chassis, which in this case is
-a _failure domain_.
+a *failure domain*.
 
 To declare a bucket instance, do the following: specify its type, give it a
 unique name (an alphanumeric string), assign it a unique ID expressed as a